There are three taps of diameter 2 cm, 3 cm and 4 cm, respectively. The ratio of the water flowing through them is equal to the ratio of the square of their diameters. The biggest tap can fill an empty tank alone in 81 min. If all the taps are opened simultaneously, then how long will the tank take (in min) to be filled?
- AOption (a)
- BOption (b)
- COption (c)
- DOption (d)
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Correct answer
Why this answer is correct
Shortcut Trick
Ratio of flow efficiency (E) = d2 = 22 : 32 : 42 = 4 : 9 : 16
Total capacity of tank = E of biggest tap × Time = 16 × 81 = 1296 units
Total efficiency of all taps = 4 + 9 + 16 = 29 units
Required time = Total capacity ÷ Total efficiency = 1296 ÷ 29 = 44 20/29 min
∴ The correct answer is 44 20/29 min.
Alternate Method
Given:
⇒ Diameters of taps = 2 cm, 3 cm, 4 cm
⇒ Flow rate (Efficiency) ∝ (Diameter)2
⇒ Time taken by biggest tap (4 cm) = 81 min
Formula used:
⇒ Efficiency1 × Time1 = Total Work (Capacity)
⇒ Time = Total Work ÷ Combined Efficiency
| Person | Days | Total Work | Efficiency |
|---|---|---|---|
| Tap A (2cm) | 324 | 1296 | 4 |
| Tap B (3cm) | 144 | 9 | |
| Tap C (4cm) | 81 | 16 |
Calculations:
⇒ Efficiency of taps A, B, C = 22, 32, 42 = 4, 9, 16
⇒ Total Work = Efficiency of C × Time of C = 16 × 81 = 1296 units
⇒ Time for tap A = 1296 ÷ 4 = 324 min
⇒ Time for tap B = 1296 ÷ 9 = 144 min
⇒ Combined Efficiency = 4 + 9 + 16 = 29 units
⇒ Total Time Taken = 1296 ÷ 29 = 44 20/29 min
∴ The correct answer is 44 20/29 min.
Additional Information
Rate of Flow and Diameter
The flow rate or efficiency of a pipe is directly proportional to its cross-sectional area, which in turn is proportional to the square of its diameter (Rate ∝ D2).
Net Efficiency Rule
When multiple pipes work together, the net efficiency is the sum of their individual efficiencies. Filling time = Capacity ÷ (E1 + E2 + ... + En).
Efficiency and Time Relationship
Efficiency is inversely proportional to the time taken. If the ratio of efficiencies of two pipes is x : y, then the ratio of time taken by them to fill the same tank is y : x.







































