Explain very briefly how you would check for divisibility by 45?

Since 45 = 9x5, a positive integer is divisible by 45 
if and only if it is divisible both by 5 as well as by 9.

Step 1. Check for divisibility by 5 by checking to see that the last digit
   is either 0 or 5.

if that is not the case then the integer is not divisible by 45. If
that is the case then go to step 2:

Step 2. Check for divisibility by 9 by adding its digits, if you get an 
   integer with more than two digits, add the digits of that integer.  If
   again you get an integer with more than two digits, add the digits 
   of that integer. Keep adding the digits of what you get until you 
   end up with just a one-digit number. (This process can sometimes be 
   shortened by omitting any 9's in any of the addition.)

If the one-digit number you end up with is not a 9, then the integer is not
divisible by 45.  If it's 9 then it is.  

Edwin