input is time, this is your x-values
output is the total cost, this is your y-values.
in the question above "this mapping diagram shows...." so i'm not sure if there more to the question or what the question actually is, but if you have to create a mapping, you will create a list of x values for days and y values for corresponding total cost.
Day 1 cost $40 (1,40)
Day 2 cost $80 (2, 80) ($40 per day for 2 days = 40 (2) = $80
Day 3 cost $120 (3, 120) ($40 per day for 3 days = 40(3) = $120
hope this answers your question. :-)