I want to calculate the doubling time of coronavirus notifications, having a daily multiplier value of 1.24.
In other words, if the number of infections increases at 24% per day, how many days does it take for the number to double?
Other replies indicate a solution using log base 2, but I don't know how to solve this function.
You need to solve $1.24^n = 2$ and you can do that by taking logarithms of both sides.
$1.24^n = 2$
$\log 1.24^n = \log 2$
$n\log 1.24= \log 2$ (using the rules of logarithms)
$n = \frac{\log 2}{\log 1.24} \approx 3.22$.
At day $3$, it won't quite have doubled, at day $4$, it will have overshot. If you're looking for an integer number at which it's at least doubled, the answer is $4$.
It doesn't matter which base of logarithm you use, as long as it's the same throughout. You can use either LOG (base $10$) or LN (base $e$) on your calculator.