Question 1032436
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Tasha bought salads at $2.75 each and cartons of milk at $.80 each. The total cost was $16.15. How many of each did Tasha buy? 
I will be using the variable s for the number of salads and m for the cartons of milk. My equation is 2.75s + .80m = 16.15.
I would like some help on how to solve the problem using this equation.
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Your equation is correct, and the problem has a unique solution.
It may seems to be strange: one equation, two unknowns and the unique solution?


Nevertheless, it is true. 

The key to resolve this mystery is the fact that you are looking for solutions among integer s and m only. 
It is very strong restriction, which replaces the second equation in this case.


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So, solve it in this way.

Take s = 1. Calculate 16.75 - s*2.75 = 13.40. Divide it by 0.8. Will you get the integer quotient?  - No, in this case.

Take s = 2. Calculate 16.75 - s*2.75 = 10.65. Divide it by 0.8. Will you get the integer quotient?  - No, in this case.

Take s = 3. Calculate 16.75 - s*2.75 = 7.90. Divide it by 0.8. Will you get the integer quotient?  - No, in this case.

. . . . . 

Check s = 5. !!!!! You will get the integer quotient m = 3.

So, s = 5 and m = 3 is your solution.
</pre>

You have managed to find the unique solution of the problem described by one equation with two unknowns.
It became possible only because you were looking for integer solutions.

Congratulations !