b)

Height of water in Tank X at first
= 18000cm³ ÷ 40cm ÷ 25cm
= 18cm

As Tank Y is empty at first, this means the height of water in Tank X is 18cm higher than in Tank Y at first.

Since it fills at 3 l /min,
and 3l = 3 x 1000cm³
= 3000cm³,

Rate that the water height increases for Tank X

= 3000cm³/min ÷ 40cm ÷ 25cm
= 3cm/min


For Tank Y, it fills at 2.1 l /min,
and 2.l = 2.1 x 1000cm³= 2100cm³


Rate that the water height increases for Tank Y

= 2100cm³/min ÷ 20cm ÷ 15cm
= 7cm/min


7cm/min - 3cm/min = 4cm/min


So every minute, Tank Y's water height is increasing by 4cm more than Tank X's water height is increasing.


This tells you that the difference in their water height is getting smaller by 4cm every minute.

Since Tank X 's water height is 18cm higher than Tank Y's at first,

18cm ÷ 4cm per minute = 4.5 min

4.5 minutes is needed for the water height in Tank Y to catch up and be the same with Tank X (whereby the difference is 0)