120 CHAPTER 1 Equations and Inequalities 21. Distance to Work David gets to work in 20 min when he drives his car. Riding his bike (by the same route) takes him 45 min. His average driving speed is 4.5 mph greater than his average speed on his bike. How far does he travel to work? 22. Speed of a Plane Two planes leave Los Angeles at the same time. One heads south to San Diego, while the other heads north to San Francisco. The San Diego plane flies 50 mph slower than the San Francisco plane. In 1 2 hr, the planes are 275 mi apart. What are their speeds? 23. Running Times In the Apple Hill Fun Run, Mary runs at 7 mph, Janet at 5 mph. If they start at the same time, how long will it be before they are 1.5 mi apart? Start Mary Janet 1.5 mi 24. Running Times If the run in Exercise 23 has a staggered start, and Janet starts first, with Mary starting 10 min later, how long will it be before Mary catches up with Janet? 25. Track Event Speeds At the 2008 Summer Olympics in Beijing, Usain Bolt set a new Olympic and world record in the 100-m dash with a time of 9.69 sec. If this pace could be maintained for an entire 26-mi marathon, what would his time be? How would this time compare to the fastest time for a marathon, which is 2 hr, 3 min, 23 sec, set in 2013? (Hint: 1m≈3.281 ft.) (Data from Sports Illustrated Almanac.) 26. Track Event Speeds On August 16, 2009, at the World Track and Field Championship in Berlin, Usain Bolt set a new world record in the 100-m dash with a time of 9.58 sec. Refer to Exercise 25 and answer the questions using Bolt’s 2009 time. (Data from Sports Illustrated Almanac.) 27. Boat Speed Callie took 20 min to drive her boat upstream to water-ski at her favorite spot. Coming back later in the day, at the same boat speed, took her 15 min. If the current in that part of the river is 5 km per hr, what was her boat speed? 28. Wind Speed Anthony traveled against the wind in a small plane for 3 hr. The return trip with the wind took 2.8 hr. Find the speed of the wind to the nearest tenth if the speed of the plane in still air is 180 mph. Solve each problem. See Example 3. 29. Acid Mixture How many gallons of a 5% acid solution must be mixed with 5 gal of a 10% solution to obtain a 7% solution? Strength Gallons of Solution Gallons of Pure Acid 5% x 10% 5 7% x + 5 30. Acid Mixture A student needs 10% hydrochloric acid for a chemistry experiment. How much 5% acid should she mix with 60 mL of 20% acid to get a 10% solution? Strength mL of Solution mL of Pure Acid 5% x 20% 60 10% x + 60
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