25% increase. The formula would be difference over original is equivilent to percent over 100. So, 12.5 over 50 is equal to x over 100. Once you cross multiply you get 1,250 = 50x. Divide and get x = 25%. To check you can plug in 12.5 over 50 = 25 over 100. You know it's correct because you get 1,250 = 1,250.