Answer:
GCF: 20
Explanation:
The thing you want to do is factor both numbers down to their lowest primes.
40:2 * 2 * 2 * 5
60:2 * 2 * 5 * 3
Now underline what is in both sets of primes. I'll bold them. The number that is bolded comes from 2 * 2 * 5 which is 20
The highest common factor is 20