Multiplicative inverse of a-b is?

Olivia uses the work below to determine 55% of 720 which explains the error in olivia’s solution

If 1.4% of the mass of a human body is calcium, how many kilograms of calcium are there in a 165-pound man? 1.0 kg ca 5.1 kg ca 1.0 x 102 kg ca 5.1 x 102 kg ca

Bob the builder is making 480kg of cement mix. this is made by cement, sand and gravel in the ratio 1: 3: 4