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