y=40x+b, we know that he has $40 on the third month (February) so:
-80=b so the equation is:
y=40x-80 (note that the domain is x=[3, +oo) only for the function to have meaining)
Now you want to know when he has $320...
So he will have $320 on the 10th month, which is October.
2 over 5 and 4 over 15
don’t know last one sorry