125.
16 (10 and 6) divided by 100 is 6.25. Times two is 12.5. (For 200) times 10 for 10 marbles.
Percentage increase = Final - Initial / Initial * 100
% increase = 500 - 250 / 250 * 100
% increase = 250 / 250 * 100 = 100
In short, Your Answer would be 100%
Hope this helps!