Solve the problem.
-Martha can rake the leaves in her yard in 4 hours. Her younger brother can do the job in 7 hours. How long will it take them to do the job if they work together?
A) $7 \mathrm{hr}$
B) $\frac{28}{3} \mathrm{hr}$
C) $\frac{11}{28} \mathrm{hr}$
D) $\frac{28}{11} \mathrm{hr}$

Question

Quizplus · Accepted Answer

Martha's rate is $1/4$ of the job per hour, and her brother's rate is $1/7$ of the job per hour. Together, they can do $(1/4 + 1/7) = (7/28 + 4/28) = 11/28$ of the job per hour. To find the time it takes to complete 1 job, take the reciprocal of $11/28$, which is $28/11$ hours.

Solve the Problem 7hr7 \mathrm{hr}7hr B) 283hr\frac{28}{3} \mathrm{hr}328​hr C) 1128hr\frac{11}{28} \mathrm{hr}2811​hr D) 2811hr\frac{28}{11} \mathrm{hr}1128​hr

Solve the Problem $7 \mathrm{hr}$ B) $\frac{28}{3} \mathrm{hr}$ C) $\frac{11}{28} \mathrm{hr}$ D) $\frac{28}{11} \mathrm{hr}$