blank 1: 4 months blank 2: 13 stamps explanation : for Joy add 2 stamps each month ( 2 times 4 equals 8 stamps) add 8 stamps to original 5 stamps ( 8+5=13) for Ronald subtract 3 stamps each month (3 times 4 equals 12 stamps) subtract 12 stamps from original 25 stamps ( 25-12=13) both end up with 13 stamps left over a time period of 4 months.