Impossible?

I'm not keen on my mathematical acumen so when I saw this on line "Can you solve this almost impossible problem?" I almost passed it over. But just by glancing I knew what I thought was the answer, so I went to the site and I was right.

It just seems common sense and intuitive to me. What do you think? Here it is....

1+5=18
2+10=36
3+15=54
4+20=?

What do you think?

(What was harder than the problem was typing it! My computer didn't let me put spaces between the numbers and signs like I wanted. Now that seems impossible....