Here's a good method to quickly find GCFs:
(1) Replace the larger number with the difference of the two numbers.
(2) If the two new numbers are the same, then that gives the GCF. Otherwise repeat step (1) on the new numbers.
With 48 and 28:
(48,28) -> (20,28) (48 has been replaced with 20 since 48-28=20)
(20,28) -> (20,8) (28-20 = 8)
(20,8) -> (12,8)
(12,8) -> (4,8)
(4,8) -> (4,4)
The two numbers are now the same, so the answer is 4.
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