# Distance in last 20 minutes

1. Jun 19, 2016

### zak100

1. The problem statement, all variables and given/known data
Stan drove for h hours at a constant rate of r miles per hour. How many miles did he go during the final 20 minutes of his drive?

2. Relevant equations

S= V* t
3. The attempt at a solution
Let H=6 hours, r=10 miles/h

1 H = 60 min

6 H = 360 min

S=v t

S= 10 * 360 = 3600miles

S= 10 * (360-20)= 3400 miles

Miles in last 20 minutes = 3600 – 3400 = 200 miles

But this we can get directly by

S= 10 * 20 = 200 miles.

I cant understand some body please guide me.

Zulfi.

Last edited by a moderator: Jun 19, 2016
2. Jun 19, 2016

### SammyS

Staff Emeritus
Several things you have written don't make sense.

Most importantly, speed and time need to be in units which are consistent with each other.

Simply change 20 minutes into units of hours.

Beyond that, the statement of the problem says nothing about total time nor about a specific speed (rate).

3. Jun 19, 2016

### Staff: Mentor

You're making this much harder than it actually is. A similar question would be "How far did he go in the last hour of his drive?" If you can answer that, you should be able to answer the question of this problem.

4. Jun 20, 2016

### zak100

Hi,
Okay I am converting everything into hours.

Let H= 6 & r (= v) = 10 miles/hour

S= v * t = 6 * 10 = 60 miles / hour

60 minutes = 1 H

1 min = 1/60 H

20 min =1/3 H

For 20 min

S = v * t = 10 * 1/3 = 3.33 miles

In Last 3 minutes, he covered = 60 -3.33 = 56 miles

Zulfi.

5. Jun 20, 2016

### zak100

Hi,
I think my solution is right. Because I have written:
S = 10/3 & i have supposed
r = 10.
So i can write:
S = r/3.

Zulfi.

6. Jun 20, 2016

### Ray Vickson

You are not allowed to assume any specific value for $r$. However, $r/3$ is correct for this problem; do you see why?

7. Jun 21, 2016

### zak100

Hi,
<You are not allowed to assume any specific value for r>

Book has solved several such questions by supposing values.
<
However, r/3 is correct for this problem; do you see why?

>

No I don't see. We have to find last 20 minutes drive i.e from 340 minutes to 360 minutes according to my assumed values. May be this is the reason you are against supposing values.

Zulfi.

8. Jun 21, 2016

### Ray Vickson

I am very much against you assuming any values, because when you do that you are NOT LEARNING! You WERE given values: h hours at a rate of r miles per hour--those are your values, end of story. Since you are supposed to be doing algebra, not arithmetic, you should not use numbers at this point. (However, you were given one number: 20 miinutes.)

9. Jun 21, 2016

### Staff: Mentor

Repeating what Ray said, the only numeric constant in this problem was the 20 minutes.

Here is the problem as you wrote it in post #1. All of the information needed is here. You should NOT make any assumptions about specific values for the speed or time.

In post 3 I said this:

10. Jul 11, 2016

### James R

zak100:

In your attempt at the solution, you used the equation s=Vt, and you plugged in V=10 miles/h. But that 10 is just a number you made up. Nowhere does the problem statement say that r=10 miles/h. It just gives the speed as r.

As for the time, t, you started off by saying that t=360 minutes, or 6 hours. But again, that 6 hour time just a number you made up. It is not stated in the problem.