Across-country skier traveled at a rate of 40 miles per hour on downhills and 20 miles per hour on flat terrain while skiing a total 54 miles. if he skied for 2 hours, how many miles of downhills did he ski?

So you know distance=rate(time). You could use the equation 40(x)+20(2-x)=54. After simplifying, you get x=7/10 or 0.7 hrs. Btw, x represents time downhill, while 2-x represents time on flat terrain since the total is 2. If you don't want to read all that, the answer is 0.7 hrs.