Question 785262
One job is, "fill the pool".  We want to use rates in units of jobs per hour.


Combined Pipes fill rate, {{{1/4.8}}} jobs per hour.


Small pipe rate, {{{1/x}}} jobs per hour.
Larger pipe rate, {{{1/(x-4)}}} jobs per hour.
These two must be well understood before continuing....


The fill rates for the two pipes open at same time are directly additive, so the equation for this is 
{{{highlight(1/(x)+1/(x-4)=1/4.8)}}} and you just solve the equation for x, the time needed for the small pipe to fill the pool by itself.