#### Answer

$2\times2\times5$

#### Work Step by Step

The first step is to write 20 at the top of your factor tree. The next question is what 2 numbers multiply together to get 20. For this example, I used 10 and 2, but it can be solved with the same method using 4 and 5. So, 10 and 2 are written under the 20 as 2 different "branches."
For each new number "branching" out from the previous number, you must determine if it is prime. 2 is prime because nothing multiplies to 2 but itself. Circle the number 2 to show its separation from the rest of the factor tree. 10, however, is not prime because 5 and 2 are another set of factors for 10 besides 10 itself.
Now, 5 and 2 are both prime, so circle each number separately .
Looking at all 3 circled numbers,$2\times2\times5$ would be the prime factorization for 20.