A Firm Produces Two Products, X and Y, and the Production

A firm produces two products, x and y, and the production process is such that one unit of x is always obtained with one unit of y. If the demand for x and y are estimated to be:
Q_x = 100 - P_x so that MR_x = 100 - 2Q_x)
Q_y = 220 - P_y so that MR_y = 220 - 2Q_y)
And the marginal cost of production is MC = 50 + .5Q_j, where Q_j consists of one unit of each product, how much of product x should the firm sell in order to maximize profit?

A) 30
B) 40
C) 50
D) 60
E) 70