Let x = number of nails costing $7 per pound. The total cost of the mixture equals the sum of the cost for each type of nail or M = A + B, where A = 7x, B = 3(6), and M = 4(6 + x). Substitute the known values into the equation. 4(6 + x) = 7x + 18.
24 + 4x = 7x + 18
24 – 18 = 7x – 4x
3x = 6
x = 2