Starting from gome, a car is driven at constant velocity of 30 m/s for 5 minutes. The driver then stops at the store. She is in the store for 10 minutes.

Question

Continued: She then drives back home at 20 m/s.

 

How many seconds have elapsed from time she left home until she arrives back

Answer 1

t₁ = time to get to store = 5 min = 300 s

t₂ = time at store = 10 min = 600 s

t₃ = time to get back home = ?

 

t = t₁ + t₂ + t₃

 

v₁ = speed on way to store = 30 m/s

 

v₃ = speed driving home = 20 m/s

 

t₃ = d/v₃

 

Get distance d from t₁ and speed v₁.

 

d = v₁t₁ = (30 m/s)(300 s) = 9000 m

 

t₃ = (9000 m)/(20 m/s) = 450 s

 

t = (300 + 600 + 450) s = 1350 s

 

Answer 2

5 minutes + 10 minutes + X minutes where X is the # of minutes on the return trip.

 

distance = rate x time

 

               30 m/s x 5 minutes = 20 m/s x T

 

               30 m/s x 600 sec  = 20 m/s x T

 

                30 x 600 / 20 = T

 

                18000/ 20 = T

 

                  900 = T

 

              900 seconds = 900/60 min = 90/6 min= 30/2 = 15 min

 

             

The entire time is 15 + 10 + 5 min = 30 min

 

That is 1800 seconds plus whatever time it took her to go in and out of the store.