Question

It took Christine the same time to travel 10km at x km/h as it did to travel 15 km at a speed of 3 km/h faster. Find x

Answer 1

Time = Distance/Speed

10/x = 15/(x+3) . . . . . times at the two (distance, speed) pairs are the same

Multiplying by x(x+3), we have
  10(x+3) = 15x
  30 = 5x
  6 = x