Blog

The Quest for 700: Weekly GMAT Challenge (Answer)

Yesterday, Manhattan GMAT posted a GMAT question on our blog. Today, they have followed up with the answer:

First, we set up two RT = D equations, one for the downhill run and one for the ride back up the mountain.

Downhill run: (x + 10)t = 300

Ride back up: (x – 8)(t + 135) = 300

Technically, we just have to do some algebra & arithmetic from here on out. However, these equations are very difficult to solve in their current state. The tip off for you is that the variable x does not represent, on its own, either the downhill or the uphill speed. Thus, the equations wind up being thorny (although still solvable).

However, we can reduce the complexity by creating a new variable, say r, that represents the speed on the ride back up. In other words, r = x – 8. We can rewrite this equation as r + 8 = x, and thus the downhill speed, x + 10, can be re-expressed as r + 18. As you’ll see, this simplifies the algebra. In this sort of situation, when a variable such as x does not represent any real speed in the scenario, our instinct should be to replace x with another variable that does represent a real speed.

Downhill run: (r + 18)t = 300

Ride back up: r(t + 135) = 300

Now we can set the expressions on the left side equal to each other, since they both equal 300:

(r + 18)t = r(t + 135)

rt + 18t = rt + 135r

18t = 135r

2t = 15r

t = (15/2)r

Finally, we substitute back into either equation (we’ll just pick the first). Since the numbers get large and we can see we’re going to get a quadratic, we might want to leave certain numbers factored as we go.

(r + 18)(15/2)r = 300

(15/2)r2 + 135r = 300

(15/2)r2 + 135r – 300 = 0

Now divide by 15 and multiply throughout by 2.

r2 + 18r – 40 = 0

(r + 20)(r – 2) = 0

Since r must be positive (it represents a speed), r must be 2. Thus, Lindsey’s downhill speed, in meters per second, is r + 18 = 20.

The correct answer is (C).



onTrack by mbaMission

A first-of-its-kind, on-demand MBA application experience that delivers a personalized curriculum for you and leverages interactive tools to guide you through the entire MBA application process.

Get Started!


Upcoming Events


Upcoming Deadlines

  • Dartmouth Tuck (Round 1)
  • IESE (Round 1)
  • MIT Sloan (Round 1)
  • Carnegie Mellon Tepper (Round 1)
  • Duke Fuqua (Round 1)
  • Georgetown McDonough (Round 1)
  • Oxford Saïd (Round 2)
  • Penn State Smeal (Round 1)
  • UCLA Anderson (Round 1)
  • USC Marshall (Round 1)
  • UW Foster (Round 1)

Click here to see the complete deadlines


2024–2025 MBA Essay Tips

Click here for the 2023–2024 MBA Essay Tips


MBA Program Updates

Explore onTrack — mbaMission’s newest offering allowing you to learn at your own pace through video. Learn more