Question 381486
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Hi,
Lettiing x and (x+7)represent the time it takes Norman and Bob alone to do the job.
Question States:  Per hour the Equalizer
  1/x + 1/(x+7) = 1/12  Multiplying each term by 12x(x+7) so as all denominators= 1
 12(x+7) + 12x = x(x+7)
 12x + 84 + 12x = x^2 + 7x
 x^2 -17x - 84 = 0
factoring
(x-21)(x+4)= 0 
 (x+4) = 0
    x = -4  tossing negative solution out
 (x-21) = 0
    x = 21 hrs, time it takes Norman.  Bob 28hrs  ( 21+7)
CHECKING our Answer
 1/21 + 1/28 =  4/84 + 3/84 = 7/84 = 1/12