Answer :
On the 21st day; the 7th day of the back medication and the 3rd day of the asthma medication.
Answer: William will take both medications on every 21 days.
Step-by-step explanation:
Given: Every 3 days William has to take medication for his back. Every 7 days he has to take medication for his asthma.
To find the on what day William take take both medications, we need to find the LCM ( least common multiple) of 3 and 7.
Since 3 and 7 are both coprime numbers therefore, the LCMof 3 and 7 is given by:-
[tex]L.C.M.(3,7)=3\times7=21[/tex]
Hence, William will take both medications on every 21 days.