Data Sufficiency (DS) | Re: Is the highest common factor of (a,b) less than 10? 1) HCF (a,64)=4 2)

From S1:

HCF(a,64) = 4
So a contains a common factor 4.
a can be 4, 12, 20, 28, 44 and so on.
No info about b.
INSUFFICIENT.

From S2:

HCF(b,64) = 1
There are no common factors between b and 64.
b can be 3, 5, 7, 11, 15, 21 and so on.
No info about a.
INSUFFICIENT.

Combining both:
If (a,b) = (4,3), then HCF = 1
If (a,b) = (60,15), then HCF = 15
Yes or no.
Hence INSUFFICIENT.

E is the answer.

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