Solving word problems: mixing two types of feed

[sta_bloo]

Tim and Judy mix two kinds of feed for pedigree dogs. They wish to make 14 lbs of feed worth $0.37 per lb by mixing one kind worth $0.11 per lb with another worth $0.85 per lb. How many lbs of the cheaper kind should they use in the mix? Round to the nearest whole lb if necessary.

x + y = 14
11x + 85y = 518

multiply by -11 to cancel out 11x

-11x - 11y = -154
11x + 85y = 518
74y = 364

y = 5

x + 5 = 14
x = 9
9 + 5 = 14

9 + 5 = 14
11(9) + 85(5) = 524 not 518

Where am I messing this up?

 

[tkhunny]

Round to the nearest whole lb if necessary.

There is the problem. Don't do that. What are the criteria for determining "necessary"?

 

[sta_bloo]

this was a homework question and the multiple choice answers are..

a) 5lbs
b) 9lbs
c)11lbs
d) 10lbs

We are in the section Systems of Equations in Two variables
substitution method and elimination method..

 

[stapel]

...Round to the nearest whole lb if necessary....

Where am I messing this up?

If you round, especially in the middle of the computations, then it is highly unlikely that you will get an exactly-correct answer.

Eliz.

 

[sta_bloo]

so 9 being the best answer out of them because it gets the equation to the closest of the answer?

 

[Denis]

74y = 364
y = 5

WHY are you stuck here?
364 / 74 = 4.9189...
Rounded to nearest: 5
So answer is 5: keep it simple :idea:

 

[tkhunny]

so 9 being the best answer out of them because it gets the equation to the closest of the answer?

This does suggest to me that you should have done one more thing FIRST! Write down clear and concise definitions. You have assumed some, but you did not write them down. I x the cheaper feed or is y the cheaper feed? If you write it down, you do not have to winder or question.