Question 1033132
The set up is like this:
{{{t/A + t/B = 1}}}
where A and B are the times working alone and t is the time working together...thus we have
{{{4/15 + 4/B = 1}}}
Multiply by 15B and get
{{{4B + 60 = 15B}}}
{{{11B = 60}}}
and
{{{B = 60/11}}} hours