So, if you do the equation C=10d+30 C is the cost, 10 is amount per day, and you have to add the 30 dollar fee. Now you take 10d or 10(5) which equals 50 and add 30. Your equation should look like this
C=10(50)+30
C=50+30
C=80
So, the final cost would be $80. Hope it helped!