The average height of a durian tree, a mango tree and a starfruit tree is 6.4 m. The starfruit tree is twice as tall as the mango tree and 0.8 m taller than the durian tree. Find the average height of the durian tree and the mango tree.
Total height of the durian tree, the mango tree and the starfruit tree
= 3 x 6.4
= 19.2 m

Height of the mango tree = 1 u
Height of the starfruit tree = 2 u
Height of the durian tree = 2 u - 0.8

Total height of the durian tree, the mango tree and the starfruit tree
= 1 u + 2 u + 2 u - 0.8
= 5 u - 0.8

5 u - 0.8 = 19.2
5 u = 19.2 + 0.8
5 u = 20

1 u = 20 ÷ 5 = 4

Total height of the durian tree and the mango tree
= 1 u + 2 u - 0.8
= 3 u - 0.8
= 3 x 4 - 0.8
= 12 - 0.8
= 11.2 m

Average height of the durian tree and the mango tree
= 11.2 ÷ 2
= 5.6 m

Answer(s): 5.6 m