Question 1193949
<pre>Here's a different approach. Think of the least common multiple 
of 7 and 3, which is 21.
</pre>
if 7 girls take 1 hour to do work how many hours will 3 girls take?
<pre>
3 times as many girls could do the work 3 times as fast or in only 1/3 of an hour.

That is, 21 girls could do the work in 20 minutes.

So 1/7 as many girls as that would take 7 times as long.

That is, 3 girls would take 20*7=140 minutes, which is 1 hour and 20 minutes.

Edwin</pre>