Renting a calculator at Hermione's school costs $1.39 for the first day and then 50 cents for any subsequent days. How much does it cost Hermione to rent a calculator for 7 days?
There will be one day that costs $1.39 and then 6 more days that each cost $0.50, so the total will be $1.39 + (6 x $0.50) = $1.39 + $3.00 = $4.39.
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