Here is a problem that can also be solved by working backward.

alex bought twice as many pears as apples. he bought 4 more pears than oranges. he bought 6 apples.

how many oranges did alex buy?

He has 6 apples, and twice as many pears as apples. 6 x 2 = 12 so he has 12 pears. He bought 4 more pears than oranges, which is another way of saying he has 4 less oranges than pears, so if he bought 12 pears then he bought 12 - 4 = 8 oranges. He bought 8 oranges.