If bill rides his bike 3 miles to fred's house and then walks with fred the remaining 1 mile to school, it will take him 30 minutes. but if he rides the entire distance, it will take him only 20 minutes. what is his walking and biking speed?

 Total distance is 4 miles. If he rides, he makes it in 1/3 of an hour (20 minutes). 

4=1/3x 

x=12 

Knowing that he bikes at 12 miles an hour, we can determine the time it will take him to go 3 miles. 

3 (miles traveled)/ 12 (miles an hour)=1/4 hour or 15 min. 

So, he will have 15 min. left to walk one mile. 

60 (minutes in an hour)/ 15 minutes per mile=4 mph.

It is the top one. the answer is -7. that is smallest