Hi, there! I'm happy to help with this. :
First scheme:
20% of 216 = (0.20)(216) = $43.20
10% of 216 = (0.10)(216) = $21.60
Cost = (20% DP) + 10*(10% MP) = 43.20 + 10*21.6 = $259.20
Second scheme8% of 216 = (0.08)(216) = $17.28
Cost = (10% DP) + 20*(8% MP) = 21.60 + 20*17.28 = $367.20
"By what percent will he find the first scheme cheaper than the second?" The second one (following the word "than") is the whole, and the first one is the part.
Percent = (part/whole)*100 = (259.20/367.20)*100 = 70.588%
The first scheme is 70.588% of the second scheme, so it is 100 - 70.588 = 29.41 = approx. 30%
Does that make it clear from whence all the numbers come? Please let me know if you have any questions.
Mike