50% of cakes baked are party cakes - 1/5 were fruit cakes and remainder were sponge cakes. what percentage of cakes were sponge cakes?

Start by think that 50% can be looked at as 5/10.
Out of the remaining 5/10, 1/5 is fruit cakes, leaving the remaining 4/5 to sponge cakes. Which means that 40% of the cakes were sponge cakes.

50% = party cakes
10% = fruit cakes
40% = sponge cakes

Change the 1/5 to 2/10. This means 20 percent were fruit cakes. 50% are party, 20% are fruit, and remainder are sponge cakes. Therefore, 30 % are sponge cakes