Let x be the amount of cornmeal to be mixed.

Then the amount of soy bean meal to be mixed is 240 - x.

The amount of protein in the cornmeal is x*6%.

The amount of protein in the soy bean meal is (240 - x)*12%.

The amount of protein in the 240 pounds mixture is 240*7%.

As the amount of protein in the cornmeal + the amount of protein in the soy bean meal = amount of protein in the 240 pounds mixture, we have:

x*6% + (240 - x)*12% = 240*7%

Solving the equation for x, we have

6x + 12(240 - x) = 7*240 (multiplying both sides by 100)

6x + 2880 - 12x = 1680

-6x = -1200

x = 200

So 200 pounds of the cornmeal should be in the mixture.