That's because you are doing the OR part of the IF instead of adding the terms. Also, there's no need for the quotation marks. This is what you should use...

=IF(C7="X",15%,0) + IF(G7="X",10%,0) + ...

i.e. IF C7 = X, add 0.15, ELSE add 0 + IF G7 = X, add 0.10, ELSE add 0