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
-----------------------------
0.35d = 70
Solve for d. 70/0.35d d = 200. Plug this number back into the original equation and c = 100.
7 pizzas, because you would divide 27 by 4 and get 6.75 and then round it up to 7.
Hope this helped :)
24.67 is just barely before 25.00, so we should be able to round 20.00, which gives us only one significant digit.