We observe that 20 is 4 more than twice 8. This suggests the time is 1 hour and the rates are
... scooter: 20 mi/h
... bicycle: 8 mi/h
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You can let b represent the rate of the bicycle in miles per hour. Then we have (for time in hours and distance in miles) ...
... time = distance/speed
... time = 20/(2b+4) = 8/b
... 20b = 8(2b+4) = 16b +32
... 4b = 32
... b = 8
The rate of the bicycle is 8 mph; that of the scooter is 2*8+4 = 20 mph.