Nvm...I got it!
so his income-tax was 66 2/3% which is the same thing as 2/3.
So let's say if his income was $60 (just picked the number) his taxe would be 60 (2/3) = $40, leaving him with $20 income.
Now his tax has reduced by 25% or 1/4, so his new tax rate would be:
(3/4) (2/3) = 1/2, so now he pays 60 (1/2) = $30 taxes, which leaves him with $30 income. Now his income has increased by $10. And $10 is 50% of $20!!!
I hope there's a shortcut!