By the way, this is another sneaky move too:

($A / 5 + $A)

If $A is a digit of 5 or greater, you need to add the two digits to get the relevant single digit number.

The calculation above doesn't do that, but just adds another 1 if the number is 5 or greater. This will give:

5 + 5 = 10 + 1(because of $A/5) = 11`
6 + 6 = 12 + 1 = 13
7 + 7 = 14 + 1 = 15

Now for the check if it's divisble by 10 it doesn't matter if you add another extra 10 to the total, so I had the same calculation as Eric, (-9 * ($A>4)) but replaced that (also today) for the calculation above.