The school is installing whiteboards in 5 classrooms. whiteboards, including their
borders, are 6 feet wide by 10 feet long. the borders, which are 0.5 ft wide, will not be
varnished. the whiteboards can be varnished to resist stains for 40 cents per square
foot or can be varnished to avoid glare from ceiling lights for 25 cents per square foot.
if each room gets one whiteboard, how much more expensive will it be to varnish the
whiteboards for stains than to varnish them for glare?

You want to know the difference in cost for varnish for 5 whiteboards with outside dimensions 6 ft by 10 ft and 0.5 ft borders, when the cost of varnish is 40 cents per square foot for stain resistance and 25 cents per square foot for glare reduction.

Area

The dimensions of the useful area of each whiteboard will be the overall dimensions less twice the border width.

height: 6 ft - 2×0.5 ft = 5 ft

width: 10 ft - 2×0.5 ft = 9 ft

Then the area of each whiteboard is ...

A = WH = (9 ft)(5 ft) = 45 ft²

Cost

The difference in cost per square foot is ...

cost for stains - cost for glare = (0.40/ft² -0.25/ft²) = 0.15/ft²

The total cost difference for 5 whiteboards is ...

(5 boards)×(45 ft²/board)×(0.15/ft²) = $33.75

It costs $33.75 more to varnish for stains than for glare.