Use the test for divisibility by 3: if the sum of the digits is divisible by 3, the number is divisible by 3. 5+1=6, so 51 is divisible by 3.

That's one where experience playing darts really helps. :-)

Where I'm from, 17 and 51 are two freeways that run kinda parallel to each other, so that was an easy one for me.