​The mean of x and y is 400, and the ratio of x to y is 3 : 7. What is the value of y – x?​

Correct option is C

Given:

Mean of x and y: $$\frac {(x + y) }{ 2}$$​ = 400

Ratio of x to y: x : y = 3 : 7

Solution:

Let,

x = 3k

y = 7k  ,where k is a constant

​$$\frac{(3k + 7k) }{2}$$​ = 400

$$\frac{10k }{2 }$$​ = 400

5k = 400

k = 80

Find x and y:

x = 3k = 3 $$\times$$​80 = 240

y = 7k = 7$$\times$$ 80 = 560​

Calculating y - x:

y - x = 560 - 240 = 320

Therefore, the value of y - x is 320.

Option (c) is right.