The average height of a persimmon tree, a durian tree and a starfruit tree is 11.4 m. The starfruit tree is four times as tall as the durian tree and 1.8 m taller than the persimmon tree. Find the average height of the persimmon tree and the durian tree.
Total height of the persimmon tree, the durian tree and the starfruit tree
= 3 x 11.4
= 34.2 m

Height of the durian tree = 1 u
Height of the starfruit tree = 4 u
Height of the persimmon tree = 4 u - 1.8

Total height of the persimmon tree, the durian tree and the starfruit tree
= 1 u + 4 u + 4 u - 1.8
= 9 u - 1.8

9 u - 1.8 = 34.2
9 u = 34.2 + 1.8
9 u = 36

1 u = 36 ÷ 9 = 4

Total height of the persimmon tree and the durian tree
= 1 u + 4 u - 1.8
= 5 u - 1.8
= 5 x 4 - 1.8
= 20 - 1.8
= 18.2 m

Average height of the persimmon tree and the durian tree
= 18.2 ÷ 2
= 9.1 m

Answer(s): 9.1 m