first crew = 1 lot in 9 hours, therefore 1 hour = [tex] \frac{1}{9} [/tex] of the lot.
second crew = 1 lot in 12 hours, therefore 1 hour = [tex] \frac{1}{12} [/tex] of the lot.
In one hour, both can seal ([tex] \frac{1}{9} + \frac{1}{12} [/tex]) of the lot, which is [tex] \frac{7}{36} [/tex]
[tex] \frac{7}{36} [/tex] = 1 hour
[tex] \frac{1}{36} [/tex] = [tex] \frac{1}{7} [/tex] hour
[tex] \frac{36}{36} [/tex] = 5[tex] \frac{1}{7} [/tex] hour
So they need 5[tex] \frac{1}{7} [/tex] hour if they work together.
