Total No. of pieces of Choc cake are x . Let the total number of pieces of White cake be y.

Hence each piece of Choc cake is 1/x of the whole cake and

Each piece of the white cake is 1/y of the whole cake.

Left over pieces of Choc cake constitute 5/x of the whole cake and the left over pieces of white cake constitute 6/y of the whole cake.

Now it is stated that left over pieces constitute ½ of a cake, which means that

5/x +6/y constitute ½ of a cake,

Or 5/x+6/y =1/2

6/y= ½ -5/x

= , which is the required fraction of the white cake left over, in terms of x

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