The average height of a durian tree, a jackfruit tree and a persimmon tree is 8.4 m. The persimmon tree is four times as tall as the jackfruit tree and 1.8 m taller than the durian tree. Find the average height of the durian tree and the jackfruit tree.
Total height of the durian tree, the jackfruit tree and the persimmon tree
= 3 x 8.4
= 25.2 m

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

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

9 u - 1.8 = 25.2
9 u = 25.2 + 1.8
9 u = 27

1 u = 27 ÷ 9 = 3

Total height of the durian tree and the jackfruit tree
= 1 u + 4 u - 1.8
= 5 u - 1.8
= 5 x 3 - 1.8
= 15 - 1.8
= 13.2 m

Average height of the durian tree and the jackfruit tree
= 13.2 ÷ 2
= 6.6 m

Answer(s): 6.6 m