I am dealing with a sample exam question that I cannot understand which is as follows:
Selection sort takes one millisecond to sort 1000 items (worst-case time) on a particular computer. Estimate the amount of time it would take to sort 100,000 items. If Mergesort took 1 second for 100 items and 15 seconds for 1000 items, how long would you expect it to take for 1,000,000?
Now, I know that selection sort has worst-case upper bound of $O(n^2)$. So as size of input $n$ increases, $f(n)$ increases in a quadratic manner. So I understand the concept of it but I don't understand how to use it to solve for how much time it would take for 100,000 items (100 times the original input). How would I solve it?