--Answer--
50 + p + 75 = 150
--Explanation--
The total number of money he had originally is 150. If he is left with 75, that starts us with 150 = 75 ______.
He spend something that was 50 dollars, so add that to 75 which equals 125.
Subtract 125 from 150 and you get 25. 25 is the cost of the single DVD, so we'll make 25 "p."
The cost of the DVD box set, the single DVD, and how much he is left with equals how much money he had originally.
That would make it 50 + p + 75 = 150.
