first off, we know the number is a multiple of 15. Meaning, 15*1, 15*2, 15*3 and so on, well, let's make a quick table.
15*1 = 15............ 1 + 5 = 6 <----- sum of its digits
15*2 = 30.......... 3 + 0 = 3 <----- sum of its digits
15*3 = 45.......... 4 + 5 = 9 <----- sum of its digits
15*4 = 60........... 6 + 0 = 6 <----- sum of its digits
15*5 = 75.......... 7 + 5 = 12 <----- sum of its digits
15*6 = 90............ 9 + 0 = 9 <----- sum of its digits
15*7 = 105............ 1 + 0 + 5 = 6 <----- sum of its digits
and is less than 100.