The greatest number that divides 43, 91 and 183 leaving the same remainder in each case is:

If a divisor leaves the same remainder each time, it divides every difference exactly. The differences are 91 − 43 = 48, 183 − 91 = 92 and 183 − 43 = 140, and HCF(48, 92, 140) = 4. Check: each of 43, 91 and 183 leaves remainder 3 on division by 4. Answering 3 reports the remainder instead of the divisor, and 12 uses only the first difference.