Question 280103
The number {{{480}}} does not need to enter the calculations
Just call washing that many dishes 1 job
In words:
(John's rate of working) + (Bob's rate of working) = rate of working together
John's rate = 1 job/80 min
Bob's rate = 1 job/120 min
{{{1/80 + 1/120 = 1/t}}}
Multiply both sides by {{{120t}}}
{{{1.5t + t = 120}}}
{{{2.5t = 120}}}
{{{t = 48}}}
It will take them 48 minutes working together