by working out 54/ 3 you either do the best stop method, how many time does 3 go into 5 which is once and you carry over the 2, making the 4 and 24 and then how many times does 3 go into 24? and the answer is 8. then you look at the total which should be 18.
or you could have a longer method and work your way up from
1x3=3
2x3=6
3x3=9
3x4=12
3x5=15
3x6=18
3x7=21
3x8=24
3x9=27
3x10=30
3x11=33
3x12=36
3x13=39
3x14=42
3x15=45
3x16=48
3x17=51
3x18=54
and just stop once you get your answer.