When a certain number is divided by 12,18,24 the reminder is always 5 find the number

Let the number be N

then

N=12a+5 = 2*2*3*a+5

N=18b+5=2*3*3*b+5

N=24c+5=2*2*2*3*c+5

so 

N-5=2*2*3*a

N-5=2*3*3*b

N-5=2*2*2*3*c

N-5 must be at least a multiple of 2*2*2 and 3*3. So 8*9=72 is a factor of N-5

In fact, it is the smallest. so the smallest N is 72+5=77

N-5 may have also other factors whose total multiplication is represented by the integer k:

N-5=72k

N=72k+5

The smallest number is 72
               The number, in general, is 72k+5, where k is a positive integer.