Let N be the number he started with
In Jan....he sold N/2 + 20
So....after this.....he had N - (N/2+20) = N/2 - 20 left
If Feb he sold (1/2) (N / 2 - 20) + 15
So
What he started with - those sold in Jan - those sold in Feb = 75
So we have
N - ( N/2 + 20) - [ (1/2) (N/2 - 20) + 15 ] = 75 simplify
N/2 - 20 - (1/4)N + 10 - 15 = 75
(1/4)N - 25 = 75
(1/4) N = 75 + 25
(1/4)N = 100 multiply through by 4
N = 400
Proof
He starts with 400
He sells (1/2)400 + 20 = 220
He has 400 - 220 = 180 left
He then sells (1/2)(180) + 15 = 105
He has 180 - 105 = 75 left