He increases his number of situps by 2 each day meaning on day 1 he'd do 2 extra, on day 2 he'd do 4 extra, on day he'd do 6 extra etc. We can show this by saying +2x where x is the day he is on. Then since he started with 20 we have to add 20 so it would be 20+2x. We want to know what day it is when he does 1100 situps so we can set our equation equal to 1100 and solve for X which is the day it would be.
20+2x=1100
2x=1080
I hope this helps and please don't hesitate to ask if there is anything still unclear!
