50% of all cakes jenny baked that week were party cakes 1/5 were fruit cakes, and the remainder were sponge cakes, what percentage of cakes were sponge cake

50% of all cakes Jenny baked that week were party cakes .50% equals to half of the cakes Jenny baked (1/2). So, 1/2 of the cakes were part cakes.
1/5 were fruit cakes. 
The remainder were sponge cakes. We can calculate how many from the cakes were sponge cakes if we eliminate from the whole (1) the amount of party cakes and fruit cakes: 1-1/2-1/5=10/10-5/10-2/10=10-5-2/10=3/10
3/10 were sponge cakes. This equals to: 30%.

You just divide and you will find x