Find amount of heads and tails from percentages
Dyalog APL, 3 bytes

,÷∨


This is a dyadic function train that accepts the percentages as left and right arguments:

┌─┼─┐
, ÷ ∨


It is equivalent to the following, train-less function:

{(⍺,⍵)÷⍺∨⍵}


Try it online on TryAPL.

How it works

  ∨  Compute the GCD of both arguments.
,    Concatenate both arguments.
 ÷   Divide the latter by the former.