How many number between 400 and 500 which is exactly divisible by 12 15 and 20?

First, find the prime factors of 12,15,20:

12=2*2*3

15=3*5

20=2*2*5

Now, compute the LCM by multiplying the factors (count duplicates once):

LCM=2*2*3*5=60

Only the multiples of the LCM will be divisible by 12, 15, and 20:

60,120,180,240,300,360,420,480,540...

{420,480}