Ron weasley isn't the best experimentalist. after an experiment he ends up with only 3.10 g of product. however, the same reaction has is a theoretical yield of 3.50 grams of product. what is the percent yield?

The experiment should theoretically yield 3.50 g of the product.
However during the experiment it can yield only 3.10 g of the product.
actual yield is lesser than the theoretical yield, therefore we need to find the percentage yield.
Percentage yield can be calculated using the following equation 

Percent yield = 3.10 g / 3.50 g x 100% = 88.6%