A part of monthly expenses of a family is constant and the remaining varies with the price of wheat. When the rate of wheat is Rs 250 a quintal, the total monthly expenses of the family are Rs 1000 and when it is Rs 240 a quintal, the total monthly expenses are Rs 980. Find the total monthly expenses of the family when the cost of wheat is Rs 350 a quintal.

Let, the part of monthly expenses which is constant be 'x' and 

The part which varies with the price of wheat be 'y'

Then according to the question,

x + 250y = 1000 ......(1)

x + 240y = 980 ........(2)

Subtract equations (1) and (2)

250y - 240y = 1000 - 980 

10y = 20 $$\Rightarrow$$ y = 2

Substitute value of 'y' in equation (1)

x + 250(2) = 1000 $$\Rightarrow$$ x = 500

The total monthly expenses of the family when the cost of wheat is Rs 350 a quintal is,

500 + 350(2) = 1200

Hence, option C is the correct answer.