Graph Y
The answer is Y because he already had built 5 layers on Monday and graph Y shows that at 0 he has 5 layers. So after 1 hour of work the total would be 15.
10(x) (layers) x (hours) + 5 (starting amount) =
To check if the graph matches you can plug in how many hours he works.
Ex: 10(4)+5 = 45