Elena purchased Brand X pens for $5.00 apiece and brand Y for $2.80 apiece. If Elena purchased a total of 12 of these pens for $42.00, how many brand X pens did she purchase?
Choose the correct answer
A) 4  B) 5  C) 6  D) 7  E) 8
4x+2.8y=42 --&gt; multiply by 2.5 (to get the integers) --&gt; 10x+7y=105 --&gt; only one positive integers solutions x=6 and y=5 (how to solve: 7y must have the last digit of 5 in order the last digit of the sum to be 5).   Answer: 6.
<result>C</result>
Elena purchased Brand X pens for $5.00 apiece and brand Y for $2.80 apiece. If Elena purchased a total of 12 of these pens for $42.00, how many brand X pens did she purchase?