Take the 5 in 15 and multiply it by 2. Write it. Then take the 5 and multiply it by 1. Write it. On the next line write a zero and then take the 1 multiplied by 2 and then 1 times 1. Here is an example... it's a little complicated to explain but here...
12
15
----
60
120
-------
180
So sorry I can't explain things well...