Multiple choice

A train covers a distance of 480 km at a speed of $x$ km/hr. If its speed is reduced to $(x - 20)$ km/hr, it takes $4$ hours more to cover the same distance. Find the value of $x$.

  1. $47$
  2. $60$
  3. $72$
  4. $80$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

480/x - 480/(x-20) = -4. 480(x-20 - x) / (x(x-20)) = -4. -9600 / (x^2 - 20x) = -4. x^2 - 20x = 2400. x^2 - 20x - 2400 = 0. (x-60)(x+40) = 0. x = 60.

AI explanation

Using the formula time equals distance divided by speed, the equation is 480 divided by x minus 20 minus 480 divided by x equals 4. Multiplying by x times x minus 20 gives 19200 equals 4x squared minus 80x, which simplifies to x squared minus 20x minus 4800 equals 0. Factoring this quadratic equation yields x equals 60 km/hr.

Keep practicing — related questions