Number of 4-digit numbers (the repetition of digits is allowed) which are made using the digits 1,2,3 and 5 and are divisible by 15, is equal to_______.

Possible combination of numbers are

Sum can be 9 or 12 or 15 or 18
 When sum is 9 

(1,2,1,5) →  3ways

 When sum is 12 

(2,2,3,5)→  3ways

 (3,3,1,5)→  3ways

(1,1,5,5)→  3ways
 When sum is 15 

(2 3 5 5)  3!=6 ways

When sum is 18
 (3, 5, 5 ,5 )→ 3 ways

Total=21 ways