The shopkeeper who takes 4 minutes will serve 75 customers over five hours and the shopkeeper who takes 5 minutes will serve 50 customers over five hours. At this rate, they will have served 125 customers together.
This can be solved by dividing the number of minutes the shopkeeper takes by the number of minutes in an hour, 60. This multiplied by five will give the answer for five hours but if you replace the five with any number it will answer for that number of hours (EX: replacing the 5 in this with a 7 will solve how many each shopkeeper can do in 7 hours). Then you should add the two totals for the two shopkeepers and you will get the total the both of them served.