...wash 60 windows in 18 minutes?
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.
5men x 30minutes/50windows = 3 man-minutes per window.
3m-m/w x 60w = 180 man-minutes
180m-m/18m = 10 men
Now the trick to doing this is to arrange fractions so the units above the fraction bar cancel units below the fraction bar. I started with "men" and "minutes" above and "windows" below and nothing canceled, so they all appeared in the answer in a form that I understood how to use. "Man-hours" is a very common unit in labor calculations, and so is "per job". The rest of the process is just arranging more fractions to get the answer the way you want it. In the last step youo might have wondered "Do I multiply or divide?" Well, the answer has to be "men", so you need the minutes below the bar to cancel minutes above the bar.
If you take a chemistry or physics class, you will be doing this process with formulas a couple blocks long.