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