Play around with RxJS: http://jsbin.com/xumejor/edit?js

rxjs_into.js

/**
 * Solving the problem:
 * In a circular race of 1200 m length, A and B start with speeds of 18kmph and 27 kmph starting
 * at the same time from the same point.  When will they meet for the first time at the starting
 * point when running in the same direction?
 */

// converts kmph to mps
function convertToMetersPerSecond(speedInKmph) {
  return (1000/3600) * speedInKmph
}

// calculates distance
function getDistanceConvered(time, speed) {
  return time * speed
}

// returnss position on track relative to the start point
function findRelativePositionOnTrack(distance, trackLength) {
  return distance % trackLength
}

// prints distance observations of the runners
function areRunnersAtSamePosition(relativeDistanceA, relativeDistanceB) {
  if (relativeDistanceA === relativeDistanceB) {
    console.log(`Runners are at the same point in ${timeLapsed} seconds.`)
    return true
  } else {
    console.log(`The two runners are at ${relativeDistanceA}m & ${relativeDistanceB}m in ${timeLapsed} seconds.`)
    return false
  }
}

// speeds in kmph
const speedA = convertToMetersPerSecond(18)
const speedB = convertToMetersPerSecond(27)

// length in m
const trackLength = 1200

// time lapsed in seconds
let timeLapsed = 0

/**
 * Creates an observer that emulates the running of a runner.
 * Emits the distance covered by the runner every second.
 * The emulator scales the time. 10 milliseconds = 1 sec.
 */
const createRunner = speed => Rx.Observable.timer(0, 10)
    .map(secondsLapsed => getDistanceConvered(secondsLapsed + 1, speed))
    .map(distance => findRelativePositionOnTrack(distance, trackLength))

const runnerA = createRunner(speedA)
const runnerB = createRunner(speedB)
const runnerObserver = runnerB.withLatestFrom(runnerA)

const runnerSubscription = runnerObserver.subscribe(distances => {
  timeLapsed++
  const [relativeDistanceB, relativeDistanceA] = distances
  if (areRunnersAtSamePosition(relativeDistanceA, relativeDistanceB)) {
    runnerSubscription.unsubscribe() 
  }
})