if the mean of a set of 12 numbers is 13 what is the sum

Let the sum of the set of numbers  = S

So....the mean would be calcuated as the sum of the set divided by its number of entries, or :

S / 12  = 13     ....... multiply both sides by 12

S  = 13 * 12   =  156  .....so.....the sum is 156