Mrs Lee had thrice as many pears as apples. The number of oranges is 1/2 the total number of fruits.

Okay. I think that what you're looking for are the fraction of the total fruits. With that being said, oranges make up 1/2 of them. That means the other half were pears and apples. There were 3 times as many pears as apples. There are still 4 parts to that left. When you think about it, 3/8 is thrice of 1/8 and 3/8 + 1/8 = 4/8 or 1/2 in simplest form. Here are the fractions of the fruits Mrs. Lee has.

Oranges: 1/2

Pears: 3/8

Apples: 1/8