It takes Mickey a total of 45 minutes to ride his bike

Rate of riding his bike =    6mi / (1/2)hr =  12miles/hr

Call his walking rate  =  W mph

And Distance / Rate  = Time....so

5 miles / 12mph  +   1mile / Wmph  =  45 min               {45 min  = 3/4 hr}

5/12 (hrs)  +  ( 1 / W ) hrs =  ( 3/4) hrs 

1/W =  3/4 -5/12

1/W  = 9/12 - 5/12

1/W=  4/12

W / 1=  12/ 4

W = 3 mph

6 / (1/2) =12 mph - his cycling speed
5 / 12 mph x 60 =25 minutes to get to Minnie's house.
45 - 25 = 20 minutes to walk to school.
1 mile / (1/3) hours = 1 x 3 =3 mph - walking speed to school.