The sum of three numbers x, y and z is 100. The average of x and y is 40 and the average of y and z is 50. What is the value of y?

Correct option is C

Given:

The sum of three numbers x, y and z is 100. 
Solution:

Average of x and y=  $$\frac{x + y}{2} = 40 \\$$​​
​$$x + y = 80 \\\text{Average of y and z: } \frac{y + z}{2} = 50 \\y + z = 100 \\x + y + z = 100 \\80 + z = 100 \\z = 20 \\y + z = 100 \\y + 20 = 100 \\y = 80$$​​