Suppose 13 oranges weigh as much as 2 mangoes , 4 papaya. Furthermore, 4 oranges and 1 mango weigh as much as 1 papaya. how many oranges weigh as much as 1 papaya?

You can solve this by using either the elimination or substitution method:

13R (oranges) = 4p + 2m

4R + 1m = 1p

XR = 1p

That's not the answer by the way. There's a lot more to it than that.