Rate problem in math

Test Preparation SAT Preparation
1

<p>Hi guys,</p>

<p>I always have problems with rate problems,but only some types like these for example I don't know how to tackle them properly.</p>

<p>If Carla drives at 40 miles per hour for n miles and then drives at 60 miles per hour for another n miles, what is her average speed,in miles per hour, for the entire trip?</p>

<p>1)42
2)48
3)50
4)52
5)54</p>

<p>I am doing these problems because they mostly is what I don't get right on the SAT,so I am solving some of these problem in order to know how to do them properly.</p>

<p>Thanks alot.</p>

2

<p>Average speed means total distance over total time.</p>

<p>Distance: n + n or 2n
Time: n miles x (1 hour / 40 miles) + n miles x (1 hour / 60 miles)</p>

<p>2n / n(1/40 + 1/60)
2 / (1/24)
48</p>

3

<p>Simple formula to remember:</p>

<p>S1 and S2 are the two given speeds. </p>

<p>2.S1.S2 / S1+S2</p>

<p>2.40.60 / 40+60
4800/100
48</p>