The average height of a mangosteen tree, a persimmon tree and a coconut tree is 8.6 m. The coconut tree is four times as tall as the persimmon tree and 1.2 m taller than the mangosteen tree. Find the average height of the mangosteen tree and the persimmon tree.
Total height of the mangosteen tree, the persimmon tree and the coconut tree
= 3 x 8.6
= 25.8 m

Height of the persimmon tree = 1 u
Height of the coconut tree = 4 u
Height of the mangosteen tree = 4 u - 1.2

Total height of the mangosteen tree, the persimmon tree and the coconut tree
= 1 u + 4 u + 4 u - 1.2
= 9 u - 1.2

9 u - 1.2 = 25.8
9 u = 25.8 + 1.2
9 u = 27

1 u = 27 ÷ 9 = 3

Total height of the mangosteen tree and the persimmon tree
= 1 u + 4 u - 1.2
= 5 u - 1.2
= 5 x 3 - 1.2
= 15 - 1.2
= 13.8 m

Average height of the mangosteen tree and the persimmon tree
= 13.8 ÷ 2
= 6.9 m

Answer(s): 6.9 m