Wayne has a recipe on a 3-inch by 5-inch index card that he wants to enlarge to 15 inches long. how wide will the enlargement be ?

That would depend on which of 3 and 5 that you consider the length.  If we see this as a standard business card the length would be 3 in as it is 5 in wide as you look at it.

w/15=5/3  multiply both sides by 15

w=75/3 in

w=25 in

...

if you need it the other way around...

w/15=3/5  multiply both sides by 15

w=45/5 in

w=9 in

Ibelieve that c. recording would work best.