If Bill rides his bike 3 mi to Freds house and then walks wi

If Bill rides his bike 3 mi to Fred\'s house and then walks with Fred the remaining 1 mi to school, it will take him 30 min. But if he rides the entire distance, it will take him only 20 min. Find his biking speed and walking speed.
for this equation, i tried to set the problems up like this:
3 b + 1 w= 30 min
b = 20 min
3(20) + 1 w = 30
60 + 1 w = 30
-60 -60
1 w = -30
I would like to see if this is a correct way to work. If not, then please assist me. I would really appreciate it!!

Solution

If Bill rides his bike 3 mi to Fred\'s house and then walks with Fred the remaining 1 mi to school, it will take him 30 min. But if he rides the entire distance, it will take him only 20 min. Find his biking speed and walking speed.
for this equation, i tried to set the problems up like this:
3 b + 1 w= 30 min
b = 20 min
3(20) + 1 w = 30
60 + 1 w = 30
-60 -60
1 w = -30
==============================
Bill DATA:
Bike:
distance = 3 mi ; rate = b mph ; time = d/r = 3/b hrs
Walk:
distance = 1 mi : rate = w mph ; time = d/r = 1/w hrs
---
Bill equation:
time + time = (1/2) hr
(3/b) + (1/w) = 1/2
============================
Bill DATA:
distance = 4 mi; rate = b mph ; time = 4/b hrs
Bill/bike equation:
4/b = (1/3) hr
b/4 = 3
b = 12 mph (speed at which Bill bikes)
----------------
Substitute into (3/b) + (1/w) = 1/2 to solve for \"w\"
3/12 + 1/w = 1/2
1/w = 1/4
w = 4 mph (speed at which Bill walks)