Suzy completes 1/100 of her homeworks per minute.
Johnny completes 1/120 of his homeworks per minute.
Together, they complete
[tex]\dfrac{1}{100}+\dfrac{1}{120}=\dfrac{11}{600}[/tex]
of their homeworks per minute. So, it takes them
[tex]\dfrac{600}{11}\approx 54.5[/tex]
minutes to complete the homeworks together.
