There are two kinds of proportions, direct and inverse.

DIRECT proportion is when both quantities go up or down together.
INVERSE proportion is when one quantity goes up, the other goes down.

This is INVERSE proportion.

The more the number of workers, the less time it takes to do a job.
The less the number of workers, the more time it takes to do a job.

So when one goes up, the other goes down.  Like a see-saw.

It's "IS TO" or DIVISION in direct proportion, but it's "TIMES" 
or MULTIPLICATION in inverse proportion.

8 laborers TIMES 28 days = 6 laborers TIMES x days:

                    8*28 = 6*x

                     224 = 6x
                   224/6 = x
                  37 1/3 = x

It would take 6 soldiers 37 1/3 days.

Edwin