Question 1115560
Rate of a painter,  {{{1/15}}}.
Rate of co-worker, {{{1/10}}}.


Painting the building is 1 job.


{{{(1/15)*3+(1/15+1/10)*x=1}}}
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{{{1/5+(2/30+3/30)x=1}}}

{{{1/5+(5/30)x=1}}}

{{{1/5+(1/6)x=1}}}, LCD is 30.


{{{30(1/5+x/6)=30}}}

{{{6+5x=30}}}

{{{5x=24}}}

{{{x=24/5}}}



If counting nearest whole days then  5 days; total of 8 days.
If counting fractions of any day, then {{{4&4/5}}}; total of {{{7&4/5}}}.