Imageine the following line chart as a calendar


begining|---------------------------------------|end
..........|
X customer|...........acquired Y customers

so (begining + throughout) / begining

looking at A we know that begining/throughout = 12z/1z
-- where z is some multiple, since we don't know if it was 12 and 1 or 24 and 2 or 36 and 3...

so (12z+z)/12z = 13z/12z = 13/12

A is sufficient

in 2 - knowing the total number does not help give proportion of each so insufficient

Hope that helps

Lets define some variables:

E = Clients at year-end
S = Clients at year-start
N = Clients acquired during the year

Clients at year-end = Clients at year-start + Clients acquired during the year

E = S + N

The question is asking for the ratio E:S.

Statement 1 gives us the ratio S:N (12:1). If there are 12 clients at the beginning of the year and 1 acquired during the year, the number of clients at year-end will be 13.

Hence, E:S = 13:12. Statement 1 is sufficient.

Statement 2 only tells us the total number of clients at the beginning of the year and without more information, we cannot calculate anything.