There is four number average of the first three is 15 and that of the last three is 16 if the last number is 19 find the first number?

Explanation
Given information:

The average of the first three numbers ( A, B, C ) is 15
frac{A + B + C}{3} = 15 A + B + C ​/3 = 15
A + B + C = 45 A + B + C = 45

The average of the last three numbers ( B, C, D ) is 16
frac{B + C + D}{3} = 16 B + C + D/3 ​ = 16
B + C + D = 48 B + C + D = 48

The last number ( D ) is 19
B + C + 19 = 48 B + C + 19 = 48
B + C = 29 B + C = 29

Finding A We already have:

A + B + C = 45 A + B + C = 45
Substituting B + C = 29 B + C = 29 :
A + 29 = 45 A + 29 = 45 A = 45 - 29 A = 45 − 29
A = 16 A = 16