One half of the birds at the pet store was yellow. Tara buys one of the yellow birds. The one third of the birds at the store are yellow. How many yellow birds were at the store before Tara brought one ?

Answer 1

x/2 - 1 = (x-1)/3
x = 4

half of 4 is 2 yellows to start with
less 1, leaves 1 yellow of 3 birds left