Basic Algebra help needed!

I've been playing Professor Layton on the DS and it threw up a algebra question, eek, which I was never good at, even at school. I had to look up the answer but it STILL doesnt make any sense so could someone just go through it briefly??

You are given the following equation but the explanation for working out C confuses me...

A+C=D

AXB=C

C-B=B

Ax4=D

They say to work out the top and bottom ones first as they both have D as the answer. It goes on to say A + C and A x 4 both equal D. Since Ax4 is the same as A+A+A+A, C must must equal A x 3. But how can it equal 3 when AxAxAxA surely equals Ax4(??).

Help will be very much appreciated!!! :)