Question 914657
Work-problems like this are a type of Uniform Rates problem.  Try this equational format:   RT=J for rate, time, amount of job.  The rates are in dimension of {{{JOBS/TIME}}} for 1 unit of time, usually.


Look at the rates for these workers.
Mason,  {{{1/12}}}
Assistant,  {{{1/x}}}
Mason+Assistant,  {{{1/12+1/x}}}


The sum of their individual rates is their rate when they work together.  We do not yet know the time needed for the assistant to do this 1 job alone; but I am calling this time, x.


The arrangement of the time and the workers was to do 1 whole job.  They work together 4 hours and then assistant works alone to finish in 10 hours.

 {{{highlight((1/12+1/x)4+(1/x)*10=1)}}}


The question essentially asks for finding the value of x.
See the way the equation uses the uniform rates rule!
{{{(RateTogether)(TimeTogether)+(RateAssistant)(TimeAssistantWorked)=ONEWHOLEJOB}}}