#### Explanation:

We could also use algebra. The tricky bit is that we need to use 6 days, not 7, because the first day is at a different price.

If we use 'd' to represent the total number of days, then the expression for the total cost will be:

Cost = $1.39 + ((d-1) x$0.50)

We could use that to calculate the cost for any number of days, but we are asked about 7 days, so if d = 7:

Cost = $1.39 + ((7-1) x$0.50) = \$4.39