When working with epoxy resin in the field, it will be required to mix small batches of?

Get a ruler in your hands. Measure things until you start to understand how a ruler works. Measure some stuff and figure out where the center is. Say you measure a book and it's 7/8" thick. You look at your ruler and see that every eighth is divided into two sixteenths, so obviously half of 7/8" is going to be 7/16". If you write that out you have 1/2 x 7/8 = 7/16. And you notice that 1/2 is divided into 2/4 and then into 4/8 and so on, so you can convert anything to anything by multiplying all the numbers on top and then all the numbers on bottom.

Other rulers are divided into 10 and 100 parts. But an inch is still an inch, so anything on one ruler can be translated to the other ruler. A half inch on one ruler is 5/10 or 50/100 on the other. An eighth inch is just 12.5 marks when you have 100 marks per inch. A metric ruler divides an inch into 25.4 parts, so a half inch would be 12.7 of those parts. Pretty simple, isn't it? Practice this a bit and people will think you went to wizard school.

100 grams resin requires 35 grams hardener. You want five times that much, so you need 500 grams resin and 175 grams hardener.

You are given a ruler with 100 marks representing the amount of resin, and 35 marks represent the amount of hardener. That is to say the hardener is 35% of the resin. Percent is just a ruler with 100 marks. So for any amount of resin, you multiply that by 0.35 to get the amount of hardener. 500 x 0.35 = 175