There are four numbers. Average of the first three numbers is 15 and average of the last three numbers is 16. If the last number is 19, the first number is equal to -

Correct option is C

Let's denote the four numbers as a, b, c and d.
Given:
1. The average of the first three numbers a, b and c is 15.
2. The average of the last three numbers b, c, and d is 16.
3. The last number d is 19.
Formula Used:
1. Average of the first three numbers: (a + b + c)/3 = 15
2. Average of the last three numbers: (b + c + d)/3 = 16
Solution:
1. From the first equation:
(a + b + c)/3 = 15
Multiplying by 3:
a + b + c = 45 ------ (1)
2. From the second equation:
(b + c + d)/3 = 16
Multiplying by 3:
b + c + 19 = 48
Subtracting 19 from both sides:
b + c = 29 ------(2)
3. Substituting equation (2) into equation (1):
a + 29 = 45
Subtracting 29 from both sides:
a = 16
The first number is equal to 16.