Let the length of the journey from home to work be x km.
`text(speed) = text(distance)/text(time)`
`text(time) = text(distance)/text(speed)`
We must use the same time units throughout. We will use hours.
` 50\ text(minutes)=50/60=5/6text(hours)`
In the forward journey, the car's time was `x/30` hours.
For the return journey, the time was `x/40` hours.
The total time was `x/30+x/40=5/6\ text(hours)`
This gives us `7x=(5xx120)/6=100`
That is `x=100/7=14.286`
So the distance from home to work is 14.3 km.