The average height of a mangosteen tree, a jackfruit tree and a durian tree is 4.2 m. The durian tree is thrice as tall as the jackfruit tree and 1.4 m taller than the mangosteen tree. Find the average height of the mangosteen tree and the jackfruit tree.
Total height of the mangosteen tree, the jackfruit tree and the durian tree
= 3 x 4.2
= 12.6 m

Height of the jackfruit tree = 1 u
Height of the durian tree = 3 u
Height of the mangosteen tree = 3 u - 1.4

Total height of the mangosteen tree, the jackfruit tree and the durian tree
= 1 u + 3 u + 3 u - 1.4
= 7 u - 1.4

7 u - 1.4 = 12.6
7 u = 12.6 + 1.4
7 u = 14

1 u = 14 ÷ 7 = 2

Total height of the mangosteen tree and the jackfruit tree
= 1 u + 3 u - 1.4
= 4 u - 1.4
= 4 x 2 - 1.4
= 8 - 1.4
= 6.6 m

Average height of the mangosteen tree and the jackfruit tree
= 6.6 ÷ 2
= 3.3 m

Answer(s): 3.3 m