Answer:
He has been doing sit ups for 9 days
Explanation:
lets do this: y = amount of sit ups, x = number of days
The equation would be:
y = 2x + 15 (let me know if you don't understand how I got this)
Now, to see how many days it took him to reach 33 sit ups, set y = 33 and plug it into the equation:
33 = 2x + 15 ⇒ subtract 15 from each side
18 = 2x ⇒ divide each side by 2
x = 9