Due to carelessness, Xiao careless miscalculated 30 * (mouth + 3) into 30 * (mouth + 3). Please help me to calculate. What is the difference between his result and the correct result? There are unknowns in the box

Due to carelessness, Xiao careless miscalculated 30 * (mouth + 3) into 30 * (mouth + 3). Please help me to calculate. What is the difference between his result and the correct result? There are unknowns in the box

eighty-seven
Subtract two numbers and cancel the unknowns. Or directly replace the unknowns with one