Question 340038
It will take the both of you working together
7 times as long as it takes the both of you 
together to wash 1 car
In words:
(your rate) + (friend's rate) = (rate working together)
Let {{{x}}} = minutes it takes with both working
{{{1/15 + 1/6 = 1/x}}}
Multiply both sides by {{{30x}}}
{{{2x + 5x = 30}}}
{{{7x = 30}}}
{{{x = 30/7}}}
This is the time it takes both working together to wash 1 car
For 7 cars, multiply by {{{7}}}
{{{7x = 7*(30/7)}}}
{{{7x = 30}}}
It will take 30 min