Physics question about speed

Question

In general a rough rule of thumb is to say that the speed of car in the city is two times the speed of biking in the city and biking is about 3 times faster than walking. If the average speed of walking is about 4km/h, how long does it take to go from Griffintown to Dawson, which is a distance of about 2.6km , by walking, by biking and by car?

 

 

I am totally lost with this. please show me steps.

Answer 1

In this problem, speed (or rate) has units of km per h.  That’s because D=R*T (Distance = Rate * Time, which you can remember as D-I-R-T) and so R=D/T.

To find how long it takes to travel a given distance (D) in a given time (T), we solve for T (T=D/R).

 

To walk the “distance of about 2.6 km” at “the average speed of about 4 km/h” takes:

        T_w= (2.6 km) / ( 4 km/h)

        T_w= 0.65 h           [note: to divide by km/h, invert to h/km and multiply so km cancels]

        T_w= 39 min

 

“The speed of biking  … is about 3 times faster than walking,” so:

        T_b= (2.6 km) / (3*4 km/h)

        T_b= ( (2.6 /(3*4) ) h ) * (60 min/h)

        T_b= 13 min

 

“the speed of car ... is two times the speed of biking,” so:

         T_c= (2.6 km) / (2*3*4 km/h)

         T_c= ( (2.6/(2*3*4) ) h) * (60 min/h)

         T_c = 6.5 min