Divide each cost by the number of pens it have to find the constant change value.
3.95/5 = 0.79
7.90/10= 0 .79
11.85/15 = 0.79
15.80 / 20 = 0.79
Since the constant change is 0.79 which means that for each pen it will cost you $0.79, then the equation will be c= $0.79p where c is the cost and p is the number of pens.
