The average height of a persimmon tree, a mangosteen tree and a durian tree is 6.8 m. The durian tree is thrice as tall as the mangosteen tree and 0.6 m taller than the persimmon tree. Find the average height of the persimmon tree and the mangosteen tree.
Total height of the persimmon tree, the mangosteen tree and the durian tree
= 3 x 6.8
= 20.4 m

Height of the mangosteen tree = 1 u
Height of the durian tree = 3 u
Height of the persimmon tree = 3 u - 0.6

Total height of the persimmon tree, the mangosteen tree and the durian tree
= 1 u + 3 u + 3 u - 0.6
= 7 u - 0.6

7 u - 0.6 = 20.4
7 u = 20.4 + 0.6
7 u = 21

1 u = 21 ÷ 7 = 3

Total height of the persimmon tree and the mangosteen tree
= 1 u + 3 u - 0.6
= 4 u - 0.6
= 4 x 3 - 0.6
= 12 - 0.6
= 11.4 m

Average height of the persimmon tree and the mangosteen tree
= 11.4 ÷ 2
= 5.7 m

Answer(s): 5.7 m