Answer :
First, yes, it is 20 - x = x + 16. Now here's the calculation to get x:
20 - x = x + 16 | + x
20 = x + 16 + x =
20 = 2*x + 16 | - 16
20 - 16 = 2*x =
4 = 2*x | / 2
4 / 2 = x =
2 = x
Jill has two marbles.
20 - x = x + 16 | + x
20 = x + 16 + x =
20 = 2*x + 16 | - 16
20 - 16 = 2*x =
4 = 2*x | / 2
4 / 2 = x =
2 = x
Jill has two marbles.