PSLE
The figure shows Tap R, Tap S and an empty tank with a capacity of 78.9 litres. Water flows Tap R at 2.3 litres per minute and from Tap S at 1.8 litres per minute. Tap R was turned on first. Tap S was turned on 4 minutes later. The taps were turned off at the same time when the tank was completely filled without overflowing. How much water flowed from Tap R?
Volume of water flowed from Tap R in the first 4 min
= 4 x 2.3
= 9.2 ℓ

Volume of water flowed from Tap R and Tap S in 1 min
= 2.3 + 1.8
= 4.1 ℓ

Additional volume of water needed
= 78.9 - 9.2
= 69.7 ℓ

Remaining time taken by both taps to fill the tank completely
= 69.7 ÷ 4.1
= 17 min

Total duration that water flowed from Tap R
= 4 + 17
= 21 min

Volume of water flowed from Tap R
= 21 x 2.3
= 48.3 ℓ

Answer(s): 48.3 ℓ