Student's solution - Ralph has 6 marbles, so Tom has 6 × 3 marbles to begin with. So Tom has 18 marbles. He lost 3, so 18 – 3 = 15, and bought 6, so 15 + 6 = 21. So Tom now has 21 marbles.

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?

Question

Student's solution - Ralph has 6 marbles, so Tom has 6 × 3 marbles to begin with. So Tom has 18 marbles. He lost 3, so 18 – 3 = 15, and bought 6, so 15 + 6 = 21. So Tom now has 21 marbles. 
 
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?

anonymous · Answer

I dont understand your top solution, but i will answer the bottom question.
The question tells us that he has bought 6 apples. 
So Apples=6.
We know he said that he bought twice as many pears as apples.
So the number of pears is 12 as 6x2=12.
It then says that he bought 4 more pears than oranges, so this means that he bought the number of pears(12) is 4 which is 8.
So Alex bought 8 oranges.