Calculate how many mg is lethal to resident Arnold who weighs 45 kg and resident Harold who weighs 96 kg. The LD50 of lead is 450 mg/kg

To get the answer, use the value that has two units because that's your conversion factor (what is used to convert).

conversion factor--> 450 mg/Kg

this value tells us that 450 mg = 1 Kg

use it so the units cancel (what you converting from in the denominator, Kg, what you converting to in the numerator, mg)

Arnold

45 Kg x (450 mg / 1 kg) = 20250 mg

Harold

96 Kg (450 mg / 1 Kg) = 43200 mg

20250mg for Arnold

43200 for Harold