RRB NTPC Preparation Series: Part 4 - Time & Work + Pipes & Cisterns in Maths

 

RRB NTPC Preparation Series: Part 4 - Time & Work + Pipes & Cisterns in Maths

Friends! Ee RRB NTPC 2025 preparation series lo Part 4 ki vacham, topic "Time & Work" + "Pipes & Cisterns" meeda. Idi Maths lo super high weightage combo –

• CBT 1 lo 2-4 questions (7-13% weightage),
• CBT 2 lo 3-5 questions (9-14% weightage),
• Total Maths lo ~25% questions (with Percentage, Profit, Interest combine).

2025 Update (Oct 28): RRB UG notification live – total vacancies 11,910. Time & Work lo LCM method, negative work, efficiency comparison ekkuva vastunnayi. Pipes & Cisterns lo leakage + inlet/outlet combo, time to empty full tank trending. Exam Jan-Feb 2026.

• Formulas (15+)
• Shortcuts (8+)
• Examples (6 NTPC level)
• Practice (12 graded)
• Integration with Part 1-3
• 2025-specific strategy

Next Part 5: Time & Distance? Adigite cheptha. Let’s conquer Maths 32+ marks! 💪

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Step 1: Introduction & NTPC Relevance

Friends, Time & Work ante "1 person 1 day lo entha work chesthadu?" – efficiency base. Pipes & Cisterns ante Time & Work ka variation – inlet fills, outlet empties.

Topic	CBT 1 Qs	CBT 2 Qs	% Weightage
Time & Work	1-2	2-3	7-10%
Pipes & Cisterns	1-2	1-2	5-8%
Combo	2-4	3-5	12-14%

Telugu: Ee topic Ratio, LCM, Percentage tho link undi. 2025 lo negative work, combined efficiency, time comparison common. English: Core concept = Work Rate. Master for 100% accuracy.

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Step 2: Core Concepts (Building Blocks)

Term	Telugu Meaning	English
Work (W)	Purna panii (1 unit)	Total job = 1
Efficiency (E)	1 day lo entha work	Work per day
Man-Days	Person × Days = Work	M×D = W
Inlet	Tank ni fill chestundi	Positive work
Outlet/Leak	Tank ni empty chestundi	Negative work

Telugu: 1 person 10 days lo 1 work → efficiency = 1/10 per day. English: Work = Efficiency × Time.

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Step 3: All Key Formulas (15+ with Telugu)

#	Formula	Telugu	Example
1	Efficiency = 1 / Days	1 person 5 days lo 1 work → E = 1/5	A: 1/10
2	Work = E × T	E × Time = Work	1/10 × 10 = 1
3	Combined Efficiency	A + B = 1/x + 1/y	A=1/6, B=1/8 → 7/24
4	Time together	1 / (A+B)	24/7 days
5	LCM Method	Total work = LCM of days	6 & 8 → LCM=24 units
6	A does x%, B does y%	A = x/100, B = y/100	60% = 3/5
7	Negative Work	Outlet = -1/x	Leak = -1/12
8	Net Rate (Pipes)	Inlet - Outlet	+1/6 - 1/12 = 1/12
9	Time to fill	1 / Net Rate	12 hours
10	Work left after t hrs	1 - (Rate × t)	1 - (1/12 × 6) = 0.5
11	A alone after B leaves	B works t1, A works t2	Standard
12	Alternate days	A + B + A + ...	Pattern
13	Wages	Work × Rate	A does 60% → 60% wage
14	Efficiency ratio	A:B = 3:2 → Time = 2:3	Inverse
15	Pipes fill in parts	2/3 fill in t hrs	t = (2/3) / Rate

English: LCM method = fastest. Negative rate for outlet.

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Step 4: Shortcuts & Hacks (NTPC Speed Boosters)

#	Shortcut	Use
1	LCM = Total Work	10 & 15 days → LCM=30 units
2	Efficiency = Units/Day	A=3, B=2 units/day
3	Time = Work / Efficiency	30 / 5 = 6 days
4	A:B efficiency = inverse days	5:10 days → 2:1 efficiency
5	Net rate = LCM / (LCM/A + LCM/B)	Direct
6	Leak time = -ve rate	-1/20 → 20 hrs to empty
7	Fill 1/2 tank	Time = 0.5 / Rate
8	A does 3/5 work	Time = (3/5) × Alone time

Telugu: LCM trick 5 secs lo solve chestundi. English: Avoid fractions – use LCM units.

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Step 5: Step-by-Step Examples (6 NTPC Level)

Example 1 (Basic - CBT 1) Q: A does a work in 10 days, B in 15 days. Together in how many days? Step 1: LCM(10,15)=30 Step 2: A=3, B=2 units/day Step 3: A+B=5 → Time = 30/5 = 6 days Telugu: LCM method best!

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Example 2 (Pipes - CBT 2) Q: Inlet fills in 12 hrs, outlet empties in 18 hrs. Together time to fill? Step 1: LCM(12,18)=36 Step 2: Inlet=+3, Outlet=-2 Step 3: Net=+1 → Time = 36/1 = 36 hrs English: Negative for outlet.

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Example 3 (Leak + Inlet - 2025 Trend) Q: Pipe fills in 8 hrs, leak empties in 12 hrs. With both, time to fill? Step 1: LCM=24 Step 2: Pipe=+3, Leak=-2 Step 3: Net=+1 → 24 hrs Telugu: Leak = outlet.

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Example 4 (A leaves early) Q: A & B together 12 days. A works 6 days, B alone finishes in 18 days. A alone? Step 1: A+B = 1/12 Step 2: A×6 + B×18 = 1 Step 3: B = 1/30 (from 18 days alone) Step 4: A×6 + 18/30 = 1 → A×6 = 0.4 → A = 1/15 → 15 days

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Example 5 (Wages - Ratio) Q: A:B efficiency 3:2, total wage Rs. 5000. A’s share? Step 1: Ratio 3:2 → A=3 parts Step 2: A = (3/5)×5000 = 3000

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Example 6 (Alternate + Part work - Hard 2025) Q: A=10 days, B=15 days. A works 1st, 3rd day; B 2nd, 4th. In 5 days, work done? Step 1: LCM=30 Step 2: A=3, B=2 Step 3: Day1: A=3, Day2: B=2, Day3: A=3, Day4: B=2, Day5: A=3 Step 4: Total = 3+2+3+2+3 = 13 → 13/30

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Step 6: Practice Questions (12 Graded)

Easy

1. A does in 20 days. Efficiency per day?
2. A & B together 12 days. A alone 20 days. B alone?

Medium 3. Pipe fills in 10 hrs, leak in 15 hrs. Together time? 4. A:B = 3:4 efficiency. Time ratio? 5. A does 2/3 work in 12 days. Full work?

Hard 6. A=12, B=18, C=24 days. A & B start, C joins after 4 days. Total time? 7. Inlet 8 hrs, outlet 12 hrs. Tank fills in 48 hrs. Capacity? 8. A works 5 days, B 3 days alternate. A starts. 60% work in how many days? 9. Pipe fills 3/4 in 9 hrs. Leak empties 1/2 in 6 hrs. Together time for full? 10. A & B fill in 6 hrs. B & C in 8 hrs. A & C in 12 hrs. All 3 time? 11. A does 40% in 8 days. B does rest in 15 days. Together? 12. Leak empties full tank in 10 hrs. Inlet fills in 8 hrs. With both, 1/3 fills in how much time?

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Step 7: Answers + Explanations

1. 1/20
2. 30 days (1/12 - 1/20 = 1/30)
3. 30 hrs (LCM=60, +6-4=+2 → 30)
4. 4:3 (inverse)
5. 18 days (2/3 work in 12 → 1 in 18)
6. 8 days (LCM=24, A+B=5, 4 days=20, left=4 → C=1 → 4 days)
7. 96 liters (Net rate=1/48 tank/hr → capacity=96)
8. 6 days (A=1/10, B=1/15, LCM=30, A=3,B=2; 5 days=3+2+3+2+3=13, 60%=18/30 → scale)
9. 36 hrs (complex, step-by-step)
10. 4.8 hrs (1/a+1/b+1/c = solve)
11. 10.8 days
12. 4 hrs (net rate calc)

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Step 8: 2025 Strategy + Common Mistakes

Strategy:

• Daily: 40 questions (RS Aggarwal Ch. 15-16).
• LCM Table: 6,8,10,12,15,18,20,24,30 memorize.
• Mock: Adda247 2025 series – Pipes section.
• Link: Efficiency = % (Part 1), Wages = Profit share (Part 2).

Avoid:

• Direct add days (wrong).
• Forget negative rate.
• LCM skip cheyadam.

Telugu: 1 week lo master, weekly 1 mock. English: Target: 0 wrong – combo topic.