Half-life is the amount of substance that decays after a certain period of time, and it is widely used to calculate the age of many things on Earth. In this question, we have 5 half-lives and 10 grams left of a substance, if we were to calculate how much would be left, we would divide the mass by 2, but since we are looking for the original mass, we need to multiply the mass left by 2:
5 half-lives = 10 grams
4 half-lives = 20 grams
3 half-lives = 40 grams
2 half-lives = 80 grams
1 half-life = 160 grams
original mass = 320 grams of the substance