Question 1121174
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After A and B worked 18 days together, they made {{{18/30}}} = {{{3/5}}} of the job;  so,  {{{2/5}}} of the job remained.


Then B completed this  {{{2/5}}} of the job in 20 days - so, his rate of work is  {{{((2/5))/20}}} = {{{2/100}}} = {{{1/50}}} of the job per day.


Thus they both have the combined rate of work  {{{1/30}}}  of the job per day working together, and the B's individual rate of work is  {{{1/50}}}. 


Hence, the A's rate of work is  {{{1/30}}} - {{{1/50}}} = {{{5/150}}} - {{{3/150}}} = {{{2/150}}} = {{{1/75}}}.


<U>Answer</U>.  A can do the job in 75 days working alone.


         B can do the job in 50 days working alone.
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Solved.