There are 36 fish in the small fish tank, which is very crowded. Some fish jumped into the big fish tank. An hour later, there were three times more fish in the big fish tank. This is how many fish are in the big fish tank.

36÷( 1+3)

=36÷4

=9 articles

9×3=27 articles

That is, there are 27 fish in the big fish tank and 9 fish in the small fish tank.