express the bigger number in terms of small number as quotient and reminder. if the reminder is zero, then the divisor is the HCF.
225 = 135 * 1 + 90, now 135 become dividend and 90 becomes divisor
135 = 90 * 1 + 45
90 = 45 * 2 + 0
as the reminder is zero, the last divisor, 45 is the HCF.
If reminder is never zero, and is finally equal to 1, then there is no HCF other than 1.
