Question

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?

Answer 1

Let x be the number of oranges, y be the number of pears and z be the number of apples.

y = 2 z
y = x + 4
z = 6 (apples)

All you have to do is substitute.
y = 12 (pears)
x = y-4 = 8 (oranges)

Answer 2

45