Question 1141747
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(1) Formally, using algebra....<br>
The thing we are to find is the number of dogs, so let that be our variable:<br>
let x = # of dogs
then x-3 = # of people<br>
The total number of legs -- 4 per dog and 2 per person -- is 84:<br>
{{{4(x)+2(x-3) = 84}}}
{{{4x+2x-6 = 84}}}
{{{6x = 90}}}
{{{x = 15}}}<br>
The number of dogs is 15.<br>
(2) Informally, using logical reasoning and mental arithmetic....<br>
(a) Add 3 more people, making the numbers of people and dogs the same.  That brings the total number of legs to 84+6 = 90.
(b) Each dog-person pair has a total of 6 legs.
(c) Therefore, the number of dogs in the park is 90/6 = 15.