If 0 text messages are sent, the total cost, is:
35+0.08(0)=35.
If 10 text messages are sent, the total cost, is:
35+0.08(10)=35+0.8=35.80
If 40 text messages are sent, the total cost, is:
35+0.08(40)=35+3.2=38.20
If 70 text messages are sent, the total cost, is:
35+0.08(70)=35+5.6=40.60.