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Step-by-step walkthrough:
a.
Well a standard half-life equation looks like this.
[tex]N = N_0 * (\frac{1}{2})^{t/p[/tex]
[tex]N_0[/tex] is the starting amount of parent element.
[tex]N[/tex] is the end amount of parent element
[tex]t[/tex] is the time elapsed
[tex]p[/tex] is a half-life decay period
We know that the starting amount is 74g, and the period for a half-life is 2.8 days.
Therefore you can create a function based off of the original equation, just sub in the values you already know.
[tex]N(t) = 74g * (\frac{1}{2})^{t/2.8days[/tex]
b.
This is easy now that we have already made the function. Here we just reuse it, but plug in 2.8 days.
[tex]N(t) = 74g * (\frac{1}{2})^{t/2.8days} = N(2.8days) = 74g * (\frac{1}{2})^{2.8days/2.8days}\\= 74g * \frac{1}{2} = 37g[/tex]
c.
Now we just gotta do some algebra. Use the original function but this time, replace [tex]N(t)[/tex] with 10g and solve algebraically.
[tex]10g = 74g * (\frac{1}{2})^{t/2.8days}\\\\\frac{10g}{74g} = (\frac{1}{2})^{t/2.8days}[/tex]
Take the log of both sides.
[tex]log(\frac{5}{37}) = log((\frac{1}{2})^{t/2.8days})[/tex]
Use the exponent rule for log laws that, [tex]log(b^x) = x*log(b)[/tex]
[tex]log(\frac{5}{37}) = \frac{t}{2.8days} * log(\frac{1}{2})[/tex]
[tex]\frac{log(\frac{5}{37})}{log(\frac{1}{2})} = \frac{t}{2.8days}[/tex]
[tex]2.8 * \frac{log(\frac{5}{37})}{log(\frac{1}{2})} = t[/tex]
slap that in your calculator and you get
[tex]t = 8.1 days[/tex]