Question 162418
Add the rates of each doing the job separately
{{{1/.5 + 1/.75 + 1/1.5 = 1/x}}}
multiply both sides by 1.5
{{{3 + 2 + 1 = 1.5/x}}}
{{{6x = 1.5}}}
{{{x = 1.5/6}}}
{{{x = .25}}}
The answer is (b) 15 min