The answer is "<span>the number of times the account compounds interest</span>".
The general formula is the following:

wherein r is the interest rate compound each four months.
Since there is 3*4 months in a year, then each year we compute the interest Three time, there where the factor 3 comes.

2) 5.625 mg will be left
Explanation:
1) Half-life = 17.5 days
initial amount of Arsenic-74 = 90 mg
To get the equation, we will use the equation of half-life:


2) we need to find the remaining amount of Arsenic-74 after 70 days
t = 70

So after 70 days, 5.625 mg will be left
Answer and work down below. Let me know if you have questions