Answer:
7
Explanation:
Your knowledge of multiplication tables tells you ...
42 = 6·7
35 = 5·7
The common factor is 7.
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You can use Euclid's algorithm to find the GCF. Find the remainder when the larger number is divided by the smaller. If that remainder is zero, the smaller number is the GCF. If the remainder is not zero, start again using the smaller number and the remainder.
42 ÷ 35 = 1 r 7 . . . the remainder is not 0
35 ÷ 7 = 5 r 0 . . . . the GCF is the divisor, 7.