-According to the sentence, 50% of all the cakes were party cakes, so: Party cakes 50%
-1 fifth were fruit cakes, so: 1/5x100= Fruit cakes 20%
-The remainder were sponge cakes: 100-50-20= Sponge cakes 30%
The percentage of cakes that were sponge cakes was 30%, as 50% were party cakes and 20% were fruit cakes.