Question 997060
Ronald's rate is unknown.  He needs some r hours to do 1 job if he works alone.  Say his rate is  {{{1/r}}}.


Mike's rate is  {{{1/5}}} jobs per hour.  He only worked alone for {{{1/3}}} hour, which is twenty minutes, before getting help from Ronald.
Both together worked for 3 hours.



{{{highlight_green(highlight_green(highlight((1/5)(1/3)+(1/5+1/r)*3=1)))}}}



---
A better choice than the variable, r, might have been.  Reminder:  I assigned r as the TIME for Ronald to do 1 job if he works alone.  Ronald's work rate is therefore  {{{1/r}}} in unit of JOB per HOUR.  A better variable choice might have been x,  or  t,  or y; but I chose r, to stand for the time for RONALD to do 1 job himself.  According to the variable assignment and the formulated equation, when you solve for r, this will be how many hours that Ronald needs to do 1 job if he works alone.