This response is coming very late, but maybe it will help. The yachts being discussed have _lengths_ of 30, 40, and 50 feet respectively. Their _areas_ are then proportional to 30^2, 40^2, and 50^2 respectively — that is 900, 1600, and 2500. And 900 + 1600 = 2500. (This assumes that the yachts are otherwise proportioned the same, so that the longer ones are also wider and taller — if they’re *just* wider then this doesn’t work.)

By way of analogy, consider a perfectly cubic room with the floor and ceiling and all the walls covered in equally-sized tiles. If you want to tile two such rooms of the same size, you will only need twice as many tiles as you would need for one. But if you want to tile a single room of twice the height (and therefore twice the length, etc.) you will need four times as many tiles. This is because the number you need doesn’t scale with the linear dimension but with the square thereof.

Now replace ‘room’ with ‘yacht’, and ’tiles’ with ‘paint’, and maybe you have a better idea of what’s happening here.

That’s not true. Because the fuel energy is not only used to accelerate the car (from 0 to 100 mph), but also used to maintain the speed. Think about elastic collision of balls in vacuum.