4 ms·

If you drive the first mile at 30 MPH and the second at 90 MPH you averaged 60 MPH ((30+90)/2). What is the math that gets "infinite speed" as the answer?

socalnate1 8y ago
There is a good explanation of this here: http://brainden.com/forum/topic/39-speeding-up/ http://brainden.com/forum/topic/39-speeding-up/
- selestify 8y ago
  Good Lord, it's frustrating to see so many people incapable of understanding such a simple thing.
idlewords 8y ago
For your method to work you need to add the time spent at speed, not the miles covered. In your proposed solution, the first mile takes two minutes, and the second mile forty seconds. That's 2:40 to cover two miles, or an average speed of 45 MPH.
celrod 8y ago
At an average of 30mph, the first mile took 2 minutes (120 seconds). At an average of 90mph, the second mile took 40 seconds. So your average speed was 2 miles / 160 seconds, or 45 mph.
MichaelDickens 8y ago
You're averaging over distance, but to calculate the average speed you need to average over time. If you drove the first mile at 30 MPH and the second at 90 MPH, you'd spend 120 seconds on the first mile and 40 seconds on the second mile, thus giving an average speed of (120*30 + 40*90) / (120 + 40) = 45 MPH
drewrv 8y ago
Helps to think about it like this: if you averaged 60 MPH on a two mile drive, your trip would last two minutes. Since your first mile was driven at 30 MPH you've already been driving for exactly two minutes. The only way to hit an average of 60 MPH for the whole trip is if the second mile is instantaneous.
pasbesoin 8y ago
Put it this way: If you were to average 60 mph over 2 miles, you would travel those two miles in 2 minutes. However, if you've already driven the first mile at 30 mph, it's taken you two minutes to drive it. In order to then average 60 mph over the full two miles, you'd have to travel the remaining mile in 0 seconds -- in zero time.

socalnate1 8y ago

There is a good explanation of this here: http://brainden.com/forum/topic/39-speeding-up/ http://brainden.com/forum/topic/39-speeding-up/

selestify 8y ago
Good Lord, it's frustrating to see so many people incapable of understanding such a simple thing.

idlewords 8y ago

For your method to work you need to add the time spent at speed, not the miles covered. In your proposed solution, the first mile takes two minutes, and the second mile forty seconds. That's 2:40 to cover two miles, or an average speed of 45 MPH.

celrod 8y ago

At an average of 30mph, the first mile took 2 minutes (120 seconds). At an average of 90mph, the second mile took 40 seconds. So your average speed was 2 miles / 160 seconds, or 45 mph.

MichaelDickens 8y ago

You're averaging over distance, but to calculate the average speed you need to average over time. If you drove the first mile at 30 MPH and the second at 90 MPH, you'd spend 120 seconds on the first mile and 40 seconds on the second mile, thus giving an average speed of (120*30 + 40*90) / (120 + 40) = 45 MPH

drewrv 8y ago

Helps to think about it like this: if you averaged 60 MPH on a two mile drive, your trip would last two minutes. Since your first mile was driven at 30 MPH you've already been driving for exactly two minutes. The only way to hit an average of 60 MPH for the whole trip is if the second mile is instantaneous.

pasbesoin 8y ago

Put it this way: If you were to average 60 mph over 2 miles, you would travel those two miles in 2 minutes. However, if you've already driven the first mile at 30 mph, it's taken you two minutes to drive it. In order to then average 60 mph over the full two miles, you'd have to travel the remaining mile in 0 seconds -- in zero time.