75lbs of food = 15% tiger's food weight
?lbs tiger= 100% tiger's body weight
So you want to take the amount of food (75lbs), divide it by the amount of the tiger's food weight (15%) and then multiply it by it's total body weight (100%)
So 75 / 15 = 5
and 5 x 100 = 500
so the tiger = 500lbs