Need For Speed. (Very Basic)

Algebra Level 2

A car travels with a speed of 20 km/hr from POINT A to POINT B. It travels at a speed of 30 km/hr from POINT B to POINT A.

Calculate the average speed through the journey.

(Do not mention the unit, just the numerical value.)

The answer is 24.

6 solutions

Prakhar Gupta
Apr 16, 2014

Let the distance from A to B is x. The time from A to B is = $\dfrac{x}{20}$

The time from B to A is = $\dfrac{x}{30}$

Total time = $\dfrac{x}{20} + \dfrac{x}{30}=\dfrac{50x}{600} = \dfrac{x}{12}$

Total distance = $2x$

Average speed = $\dfrac{Total Distance}{Total time}$

Average speed= $\dfrac{2x\times12}{x}$

Average speed = $24$

Aravind Osuru
Mar 31, 2014

avg speed =2xy/x+y

Shouldn't that be 25? I think formula for average speed is Total distance/2.

So answer must be $ \left( \frac { 20\quad +\quad 30 }{ 2 } \right) km/hr\quad =\quad \frac { 50 }{ 2 } \quad km/hr\quad =\quad 25\quad km/hr $

Aryan Gaikwad - 7 years, 2 months ago

Avg Speed = Total Dist/Total Time.........Avg Speed = (d1+d2+d3+.....dn)/(t1+t2+....tn)

V Vishnu Prasad - 7 years, 1 month ago

Can u explain to me why we have that formula to find out the answer???? Tks

Linh Tran - 7 years, 1 month ago

Avg speed = Total dist/Total time....... total dist = 2D[since dist b/w A & B is const.,assuming it to be "D", to & fro dist is "2D')...Time= Dist/Speed....T1= D/20..T2=D/30......................Avg speed =(D1+D2)/(T1+T2)........... Avg Speed = [D+D]/[(D/20) + (D/30)]= [2 D 20 30]/[30D + 20D] = [2 20*30]/[50] == 24 {N.B : AVG SPEED = 2XY/(X+Y)}...where X,Y are the given speeds...Hope its clear now...Thanks

V Vishnu Prasad - 7 years, 1 month ago