To avoid the cumbersome reference to "kg of mangoes" or "kg of durians" I am using the algebra approach to simplify this problem:
Let M represent "1kg of Mangoes"
Let D represent "1kg of Durians"
Given:
12 M --> 2/5 x 5D = 2D
1D --> 6M .......................... (1)
Given:
1M + 1D --> $3.50 ............(2)
Substitute (2) into (1)
7M --> $3.50
1M --> $3.50 / 7 = $0.50 .....(3)
Substitute (3) back into (2):
1D --> $3.50 - $0.50 = $3
Or (3) into (1):
1D --> 6 x $0.50 = $3
