Quantity before and after (respectively):

25 30

Price before and after (respectively):

15 20

Not sure why the answer would be 0.67 for elasticity of supply, rather than 0.60. Apparently, we are supposed to take the “average” of the price and quantity.

I thought the answer would be (5/25)/(5/15), which provides 0.60, but this is not the answer.

Apparently they’re using the after quantity and price as the divisors.

That’s weird.

You should use the beginning quantity and price if you’re asked simply for price elasticity of supply, and the midpoint quantity and price if you’re asked for the arc (price) elasticity of supply.

The question provides those numbers in my OP, then asks to calculate the elasticity of supply. How are we supposed to know whether to simply calculate elasticity of supply or arc elasticity of supply?

If they want arc elasticity, they’ll ask for arc elasticity.