A stack of four hundred twenty cards is placed next to a ruler, and the height of stack is measured to be 7/8 inches. How thick is one card?

7/8 divided by 420 = 7/8 x 1/420 by inverting & multiplying

= 7/3360 by multiplying tops together & bottoms together

= 1/480 inch reducing fraction by dividing top & bottom by 7

(and yes, I know that top is called numerator & bottom is called denominator, otherwise known as upstairs & downstairs)

Could also work by cross-cancelling earlier, dividing 7 by 7 = 1 and dividing 420 by 7 = 60

leaving 1 x1 on the top & 8 x 60 on the bottom. Will get same result as before, 1/480 inch.