An easier way to solve this one without any substitution: Given: X+Y=int_0 Prove: Y=int? 1) X-Y=int_1 Subtract prompt equation from eq in 1: X-Y=int_1 -(X+Y)=int_0 -2Y=int_1+int_0 This tells you that Y is the sum of two integers. Because we don't know if int_1 and int_0 are even or odd we cannot say...