a. Get the prime factorizations of 52 and 78:
52 = 2² • 13
78 = 2 • 3 • 13
Then the HCF/GCD of 52 and 78 is 2 • 13 = 26, because both 2 and 13 are common divisors of 52 and 78.
b. Do the same thing for 95 and 120:
95 = 5 • 19
120 = 2³ • 3 • 5
Then the HCF is 5.
