Volume P6 2010 SA2 HHK P2 Q 18

Tank A
6 h --> H
2 h --> 2/6 x H = 1/3 H

Tank B
4 h --> H - 5
1 1/2 h --> 3/2 x 1/4 (H - 5) = 3/8 (H - 5)

1/3 H = 3/8 (H - 5)
8 H = 9 H - 45
H = 45 cm

It was 45 cm.