How many terms of the sequence $\sqrt{1},\sqrt{2},\sqrt{3},\sqrt{4},\ldots$ are less than or equal to $20$?
I notice that this sequence is increasing, and each term is the square root of a natural number. So, to find out how many terms are less than or equal to $20$, I need to find the largest natural number $n$ such that $\sqrt{n} \leq 20$. I can do this by squaring both sides of the inequality and simplifying: $n \leq 20^2 = 400$. The largest natural number that satisfies this inequality is $n = 400$, so there are $400$ terms in the sequence that are less than or equal to $20$. 