Suppose you can fill 1/3 of a carton in 5 minutes by counting third figure the improper fraction in?

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.

You really need to spend some time talking with people so you can learn the language.

"If you can fill 1/3 of a carton in 5 minutes, how many cartons can you fill in 40 minutes?"

speed = 1/3 carton per 5 minutes
speed = 1/15 carton per minute

Now we use a trick that is used all the time in physics classes: arrange fractions so the units cancel and leave the answer in the units you want.

(1/15) carton/minute x 40 minutes =

Now check for validity. We have minutes below the fraction bar and again above so that cancels. We have carton above the bar so that appears in the answer, and that is what we want. Now multiply everything above the bar and divide by everything below.

(1/15) carton/minute x 40 minutes = 2 2/3 cartons <-- ANSWER