Person 1 can do a certain job in twenty-one minutes, and person 2 can do the same job in twenty-eight minutes. When completing the job together, which expression would be used to represent the amount of work done by person 1?

a. 21
b. 1/21
c. x/21
d. 21x

Question

Person 1 can do a certain job in twenty-one minutes, and person 2 can do the same job in twenty-eight minutes. When completing the job together, which expression would be used to represent the amount of work done by person 1?

a. 21
b. 1/21
c. x/21
d. 21x

anonymous · Answer

b? mnot sure..

shane_b · Answer

To set up the problem you add up the rates like this:$$\frac{1}{21}+\frac{1}{28}=\frac{1}{total\space if\space done\space together}$$

shane_b · Answer

So the left fraction will be would be the rate of person 1.

shane_b · Answer

IF the question provided the rates of each person and you wanted to know how fast they could do the work together you could also use this shortcut:$$\frac{r_1r_2}{r_1+r_2}=total\space if\space done\space together$$

anonymous · Answer

so b is wrong?

shane_b · Answer

No...b is right.