I believe it is pleaing i hope this is correct and helps
Add all the tees together and that is 48 so no we know the probability will be out of 48. There is 18 brown and blue tees altogether (12+6=18). Now, to work out the probability of selecting any other colour that those two we substitute; 48-18=30 and therefore the probability is 30/48 (30 out of 48)
The equivalent to this expression is 4 (21 + 4)=84+16
Hope this helps :)