You will start this experiment with 25 mg of the trans isomer. What is the percent yield if you obtain 5 mg of the cis isomer?

Isomers have the same molecular formula but they only differ in structure. So, when trans isomer is converted to cis isomer, you would expect a total mass of 25 mg exactly. So, the percent yield would be:

Percent yield = 5 mg/25 mg * 100 = 20%