How long will selection sort and merge sort take to sort a certain number of items?
They want you to apply rule of three under ridiculous assumptions. 


  If an O(n²) algorithm takes 1ms for 1000 elements, then from 1ms = c * 1000² and the ansatz Xms = c * 100000² we get that X = 10000.


In essence, they want you to assume that the running time function equals $cn^2$, in which case you could compute the constant $c$ from the given information.

The problem is that 


$O(\_)$ is only an upper bound and does not tell you anything about the true behaviour of the described function,
there are unknown lower-order terms hidden in that $O(\_)$ (and also $\Theta(n^2)$, if you had that) and
asymptotics tell us nothing for any finite $n$.
They also ignore that the algorithms take different amounts of time for different inputs of the same size and
that "time" is a famously fickly cost measure as it depends a lot on effects like caching and process scheduling.


Therefore, the problem is senseless.