This blog is managed by Song Hock Chye, author of Improve Your Thinking Skills in Maths (P1-P3 series), which is published and distributed by EPH.

## Sunday, August 22, 2010

### RGS Primary 2009 PSLE Math Prelim Paper 2 Q12

Tap A, Tap B, Tap C and an empty rectangle tank are shown below.

Lily turned on Tap A with water flowing at a rate of 5 litres per minute. After 2 minutes, she placed a rock of volume 1250 cubic cm in the tank and turned on Tap B and Tap C as well. Tap C drains the tank at the rate of 2 litres per minute. After 5 more minutes, Lily turned off all the taps and noted that the height of the water level was 30 cm. Find the rate of the flow of water from Tap B.

Solution

Tap A (water filled into tank)
--> 5 litres x 7 (min) = 35 litres

Tap B (water drained from tank)
-->2 litres x 5 (min) = 10 litres

Volume of water in tank in the end
--> (50 x 30 x 30) cubic cm - 1250* cubic cm
= (45 000 - 1250) cubic cm
= 43 750 cubic cm
* volume of stone

43 750 cubic cm is the total volume of water filled through Tap A and Tap B less the volume of water drained from Tap C.
Tap A + Tap B - Tap C --> 43 750 ml
Tap B --> 43 750 ml - Tap A + Tap C
= 43 750 ml - 35 000 ml + 10 000 ml
= 18 750 ml (after 5 min)

1 min --> 18 750 ml divided by 5
= 3750 ml/min
= 3.75 litres/min