You have to solve two equations simultaneously. The equations are:
.40c + .75d =$190
c + d = 300.
Multiply this second equation by -0.4 on both sides leaving you -0.40c -0.40d = 120. Subtract this new equation from the first and solve for d.
0.40c + 0.75d = 190
- 0.40c - 0.40d = 120
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0.35d = 70
Solve for d. 70/0.35d d = 200. Plug this number back into the original equation and c = 100.