How many phones were sold at the wrong price?
Mathematics

Tutor: None Selected  Time limit: 1 Day 
An error occurred in your bookkeeping department this month. The price of one of the smart phones that you sell is $489. Several customers were charged $565 for the phone instead. You know that the total sales for this month were $213,873 and that 401 phones were sold.
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Well, let's write down what we know
We know the price of the smart phone should 489
But several customers were charged 565
Let's say x= is the number of phones that was sold at the right price
y=is the number of phone sold at the wrong price
489x+565y=213873
But we also know that a total of 401 phones were sold, so we take the sum of x and y
x+y=401
The work:
Multiply the second equation by 489, and then subtract the two equations to solve for y
489x+565y=213873
489(x+y=401)
489x+565y=213873
489x+489y= 196089
76y=17784 (divide by 76)
y=234
Substitute y=234 into x+y=401
x+234=401
x=167
Let's do a check of our work. Substitute x and y into 489x+565y=213873
489(167)+565(234)=213873 CHECK!
Therefore the answer is
234 phones were sold at the wrong price.
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