Let the actor's initial weight be 'x'
He is then gaining 20 pounds. The clue is in the word 'gaining' as it means 'increasing' or in Mathematics can be related as 'plus'
The expression is: x+20
He then loses 15 pounds. The clue is in the word 'losses' as it means 15 pounds is to be subtracted from the last value
Last value is x + 20
The weight now is x + 20 - 15 = x + 5