you can find it with this simple way:
40 |2
20 |2
10 |2
5 |5
1
Now you have to square all the numbers and put them in roots
[tex] \sqrt{2 {}^{2} } \times \sqrt{2} \times \sqrt{5} [/tex]
and then we have:
[tex]2 \sqrt{2} + \sqrt{5} [/tex]
[tex]2 \sqrt{10} [/tex]
The answer is
[tex]2 \sqrt{10} [/tex]