You used two different formulae here. You used the new speed as the divisor in the first two cases, and then used the original one in the third, which is the only reason you got 50%. Using your method of devising increases(a wholly inaccurate one), the increase from 2 ghz to 3 is only that of 33%, as (3000-2000)/3000=33.333%.
I hate to be teaching math, since the highest I took was college algebra, but if you're using your method, you divide at the end by the original speed. So, (833-700)/700, not 833. We're doing increases. That gives you .19. His math is exact; yours is flawed.