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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?

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