Which is equivalent to 3 square root of 125

The √125 can be factored out to 25 and 5 then 25 can be factored out to 5 and 5. Then you pair up the numbers, you have a pair of 5's and a 5 left over. So the pair of 5's goes inside the radical as √5 and on the outside of the radical you have 5×3. So 3√125≈15√5.