<p>The johnsons drove to their relative’s house at 80km/h. on the way home, they reduced their speed to 10 km/h. if their total driving time was 3 hours, find the total distance driven in kilometers.</p>
<p>How do I do this???</p>
<p>Xiggi will kill me if I don’t show the fast_method first :), so here goes:</p>
<ol>
<li>Quick way: for problems like this, use the formula</li>
</ol>
<p>Avg.speed for entire trip = 2 (speed1 * speed2) / (speed1 + speed2)</p>
<p>In this case, avg.speed for entire trip = (2)(80)(10)/(80 + 10) = 1600/90 kmh</p>
<p>Round_trip distance = (total time)(avg.speed) = (3)(1600)/90 = 160/3 km.</p>
<ol>
<li>Longer solution, from scratch:
If x is the one<em>way distance in km, then
total time for round trip = x/80 + x/10 = 3
x(10 + 80)/(80 * 10) = 3
x/3 = (80 * 10) / (80 + 10)
or (2x)/3 = total distance/ total time = 2 (80)(10) / (80 + 10) = avg.speed,
which just proves the formula used in the quick</em>solution.</li>
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<p>thank you :-D</p>