Let N is the least number of six digits which when divided by 4, 6, 10, 15 leaves 3 as a remainder. The sum of all digits of N is.

6

LCM of 4, 6, 10, 15 = 60

Least number of six digits = 1,00,000

When 1,00,000 is divided by 60 the remainder = 20

So least number of six digit, which is completely divisible by 60 = 1,00,020

Therefore, N = 100020 + 3 = 100023

Sum of digits = 1 + 2 + 3 = 6