Answer:
3
Explanation:
The greatest common divisor (GCD) of two nonzero integers a and b is the greatest positive integer d such that d is a divisor of both a and b (Wikipedia)
To find out which is the GCD of 72 and 159, keep factoring both until they cannot be factored any more
Then take the factors common to both list of factors; the greatest among them is the GCD
The factors of 72 are: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72
The factors of 159 are: 1, 3, 53, 159
The common factors are 1, 3
So GCD is 3