I know this has question has been answered, but I wanted to show you this neat trick I use for cross multiplying:
For Example: 8/4=x/10
For this I would use a method called the oval circle method:
Basically you oval a diagonal with only numbers no letters and circle the remaining number. And the letter will go
as your x=. So we oval 8 and 10 and circle 4. And we make x as x=. And we plug into the formula: x=aXb/c
So it becomes x= 8 X 10/4
x=80/4
x=20.
But I suppose it wont work for this problem, but it's extremely simple and easy way to do it.