jnbimmer wrote:If n is positive integer, what is the tens digit of n?

1) The hundreds digit of 10n is 6.
2) The tens digit of n+1 is 7.

The 10s digit is the one two to the left of the decimal. We need to know what that digit is.

1) well, if the hundreds digit of 10n is 6, then the 10s digit of n must be 6.

In other words, we know that 10n can be written as 6xy (there could be digits before 6 too, we really don't care), in which x is the 10s digit and y is the units digit.

if 10n = 6xy, then
n = 6xy/10 = 6x.y

Sufficient.

2) if the 10s digit of n+1 is 7, then the 10s digit of n could be 6 or 7.

We can let n=69 or n=70 to see that we can get two different answers for our 10s digit.

Insufficient.

(1) is sufficient, (2) isn't: choose A.