This is the concept of algebra, we solve the question as follows: The cost of renting a camping tent is y The number of days is x cost is proportional to number of days, thus: yαx y=kx where k is the constant of proportionality. If y=$56 when x=7 days, then: 56=7k hence: k=56/7=8 therefore: y=kx will be: y=8x the equation in slope intercept form which is y=mx+c is y=8x. In our case c=0 because there was no initial payments required to rent apart from the daily charges.
He only read 20 minutes last saturday so that means he needs to read 10 extra minutes so he can make up for the minutes he lost. Add 10 to 30 minutes and you get 40 minutes.