PSLE
Water could flow out of a tank through tap A and tap B. At first, Julian turned on tap A. After some time, he also turned on tap B so that more water flowed out. Later, he turned off both taps at the same time. The line graph shows the volume of water in the tank from 21 00 to 21 20

1. What was the rate of flow of water from tap A in litres per minute?
2. How many litres of water flowed out of the tank throughout the period when both taps were turned on?
3. What was the rate of flow of water from tap B in litres per minute? 

(a)
Volume of the tank
At 21 00: 90 ℓ
At 21 05: 81 ℓ
At 21 10: 69 ℓ
At 21 15: 57 ℓ
At 21 20: 57 ℓ

Only Tap A was turned on from 21 00 to 21 05.

Decrease in the volume of water from 21 00 to 21 05
= 90 - 81
= 9 ℓ

Rate of flow of water from Tap A
= 9 ÷ 5
= 1.8 ℓ /min

(b)
Both taps were turned off at 21 15.

Volume of water that flowed out when both taps were turned on
= 81 - 57
= 24 ℓ

(c)
Tap A and Tap B were turned on from 21 05 to 21 15 for a duration of 10 min.

Volume of water that flowed out from Tap A in 10 min
= 10 x 1.8
= 18 ℓ

Volume of water that flowed out from Tap B in 10 min
= 24 - 18
= 6 ℓ

Rate of water from Tap B
= 6 ÷ 10
= 0.6 ℓ/min

Answer(s): (a) 1.8 ℓ/min; (b) 24 ℓ ; (c) 0.6 ℓ/min