In order to find out how many times Gary will have guitar lessons and and band practice on the same day, we have to find the least common multiple (LCM) for 5 and 4.
5 : 5,10,15,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,100
4: 4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76,80,84,88,92,96,100
So, the last common multiple for both numbers is 20, which means that every 20 days he has band practice and guitar lesson. As we can see, the number of times he has had band practice and guitar lessons in the same day out of 100 days is 5 times. Therefore, the number of times Garry will have guitar lessons and band practice is 5 times on the same day out of 100 days.