The average height of a durian tree, a mangosteen tree and a persimmon tree is 6.6 m. The persimmon tree is thrice as tall as the mangosteen tree and 1.2 m taller than the durian tree. Find the average height of the durian tree and the mangosteen tree.
Total height of the durian tree, the mangosteen tree and the persimmon tree
= 3 x 6.6
= 19.8 m

Height of the mangosteen tree = 1 u
Height of the persimmon tree = 3 u
Height of the durian tree = 3 u - 1.2

Total height of the durian tree, the mangosteen tree and the persimmon tree
= 1 u + 3 u + 3 u - 1.2
= 7 u - 1.2

7 u - 1.2 = 19.8
7 u = 19.8 + 1.2
7 u = 21

1 u = 21 ÷ 7 = 3

Total height of the durian tree and the mangosteen tree
= 1 u + 3 u - 1.2
= 4 u - 1.2
= 4 x 3 - 1.2
= 12 - 1.2
= 10.8 m

Average height of the durian tree and the mangosteen tree
= 10.8 ÷ 2
= 5.4 m

Answer(s): 5.4 m