So if you put 54 divided by 7 into a calculator you will get 7.7142857....
This means that the whole number you are looking for is 7, but you still need to find the remainder. So to do that you can look at your times tables to help you find out.
7×7=49 , 7×8 = 56 , 7×9 = ...., etc.
Now you can see that your number, 54, is between the answer for 7×7 and 7×8.
And so to find out the remainder all you need to do is count how much bigger 54 is compared to 7×7, i.e. how much bigger than 49.
So, 54 - 49 = 5
And so the answer to your problem is, 7 remainder 5 (or 7r5) 