Question 1153741
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Let x be the number of days in San Francisco and let y be the number of days at Lake Tahoe.


From the condition, you have this system od 2 equations in 2 unknowns


      x +    y =    9      (1)    (nimber of days, total)

   475x + 350y = 3650      (2)   (dollars spent, total)


To solve the system apply the Substitution method.

For it, express y = 9-x from equation (1), and substitute it into equation (2), replacing y. You will get


    475x + 350*(9-x) = 3650.


Simplify step by step and solve for x


    475x + 3150 - 350x = 3650

     125x              = 3650 - 3150 = 500

        x                            = {{{500/125}}} = 4.


<U>ANSWER</U>.  4 days in San Francisco, and the rest, (9-4) = 5 days at the Lake Tahoe.
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Solved.