If Dick could stuff envelopes working alone for six hours and Jane could stuff them all in four hours how long would it take them working toghether?

If Dick could stuff envelopes working alone for six hours and Jane could stuff them all in four hours how long would it take them working toghether?
Solution:D's rate = x/6 envelopes per hour
J's rate = x/4 "
together x = (x/6 +x/4) * Time
rewritten: x = (4x/24 + 6x/24) * time
again: x = 10x/24*Time
again: 24x = 10x*Time
again: 24/10 = Time
ergo: 2.4 hours (or 144 minutes) <<<< answer