The pronoun after 4/5 of is “it” so the total amount of rice, 100 pounds, is the amount in 1;
the fraction is 4/5; the amount in it,, the amount of rice used in cooking, is what we are looking for.
The list of the 3 elements:
the amount in 1: 100, pounds
the fraction: 4/5
the amount in the fraction: the amount of rice used in cooking?
the amount in fraction= the amount in 1 x the fraction= 100 x 4/5=80 pounds
Mom used 80 pounds of rice in cooking