So basically you want to find the total percentage of cakes that weren't sponge cakes first.
You already have the 50% of party cakes. For the 1/5 percent of fruit cakes, you can multiply it by 20/20, to get 20/100, and then just take the 20 to get 20% that were fruit cakes.
Now you can just add the percentages together.
50% + 20% = 70%
So now you know 70% weren't sponge cakes, out of 100%.
So here you can just subtract 70% from 100% to figure out the remaining part of 100%, which must be sponge cakes.
100% - 70% = 30%
So 30% of the cakes were sponge cakes.
