The formula for glue says to add 45mL of hardener to each container of resin. How much hardener should be added to 18 containers of resin?

So here we can just multiply 45 by 18, 45 mL of hardener by 18 containers of resin, because it's 45 mL of hardener for each container of resin, and there are 18 containers.

18 * 45 is 810.

So 810 mL should be added to each container of resin.