Question 614485
Your problem is poorly worded.
You can't buy 20 t shirts with 145 dollars.

y=15x+10
y=20*15+10
y=300+10
y=310
20 t shirts will cost you 310 dollars

y=15x+10
145>=15x+10
135>=15x
27>=3x
9>=x
You can buy 9 shirts for $145