the way i work it out is if you take the product and power it to the third you can easily find out which answer is correct. another way, if you don't have a calculator is to look at the last digit in the number in the square root sign.
For example, 3 square root 64 equals 8. you would look at the 4 and try to find 3 numbers that end in 4 OR is divisible three times by the number. 64 is exactly divisible by 4 only 3 times so you know that it can't be 8.
