Let,
M = # of Mangoes
A = # of Apples
Total Fruits = 20
Condition:
The mangoes are 4 times the number of the apples.
M = 4A
A = ?
If there are 20 fruits in all.
M + A = 20
solve for A,
M + A = 20
but M = 4A
4A + A = 20
5A = 20
A = 20/5
A = 4
then solve for M,
M = 4A
M = 4(4)
M = 16