Answer
17 minutes
Work Step by Step
Let both mark and lynn clean the office in $T$ hours together.
(Fraction work done in a minute by mark)+ (Fraction work done in a minute by lynn)=1
$\implies (\dfrac{1}{30}+\dfrac{1}{40})T=1$
$\implies (\dfrac{4+3}{120})T=1$
$7T=120$
or, $T=17.142$ hours
or, $T \approx 17$ minutes
