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13 ARMY AIR FORCES Solving this for x gives x=17.6 ft/sec. Therefore, 12 mph is equivalent to 17.6 ft/sec. g. Exercises.–It is customary to let x represent the unknown quantity. In the following exercises find the values of x which will make the proportions true. (1) 5/1=x/3 (2) 5/6=x/3 in. Solution: Cross-multiplying, 6x=15 inches. If 6 x's = 15 inches, then obviously, one x must be one-sixth as much for 2.5 inches. In all of these problems, if the factor multiplying x is not 1, divide by the factor on both sides of the equality sign. Thus 6x/6=15 in./6 is the same as 6x=15 inches. But 6x/6=x. Therefore x=(15 in.)/6=2.5 in. Answer. (3) 5/3=x/5 (4) 3/7=5/x x=11 2/3 Answer. (5) .25/.75=3/x (6) (1/4)/(.5)= 5 x x=10 Answer. (7) x/y=a/b (8) What is the value of A in the following proportions: (a) 17/45=14/A (b) 3/A=10/25 A=7.5 Answer. (c) 3/4 over 4/5=6/A (9) Two pulleys are connected by a belt. The smaller one runs at a speed of 750 rpm and the larger at 200 rpm. What is the ratio of their speeds? 26