Let's call the distance between Bob's house and his job " Q miles".
When he drives the distance at 60 mph, it takes him ( Q/60 ) hours to cover it.
When he drives the distance at 25 mph, it takes him ( Q/25 ) hours to cover it.
When he drives both ways, it takes him ( 1hr 8min ) total driving time.
1hr 8min = 68 minutes. That's 68/60 hours.
(Q/60 hr) + (Q/25 hr) = 68/60 hr
Q/60 + Q/25 = 68/60
Multiply each side by 60:
Q + 60Q/25 = 68
Now multiply each side by 25:
25Q + 60Q = 1700
85Q = 1700
Divide each side by 85 : Q = 20 miles
Check:
Driving 20 miles at 60 mph takes 1/3 hour = 20 minutes
Driving 20 miles at 25 mph takes 4/5 hour = 48 minutes
Driving both ways takes (20min + 48 min) = 68 minutes yay!
