Think about the squares of the numbers in the choices.
A
Practice makes perfect
We want to solve the square root. First, let's add the numbers under the square root to simplify the expression.
sqrt(121+104)=sqrt(225)
Let's now think about the definition of sqrt(225).
x=sqrt(225) ⟹ x^2=225
The question is which of 15, 21, 25, 125, and 225 gives 225 as a result when it is squared.
We can of course do this by trying all the numbers, but this takes time.
If we notice that four of these numbers are greater than 20, then we can think as follows.
y>20 ⟹ y^2>400
The square of a number greater than 20 is greater than 400, so it is certainly not 225.
This leaves us only one choice.
15^2=225 ⟹ sqrt(121+104)=15
The correct answer is A.
