Elena purchased Brand X pens for $4.00 apiece and brand Y for $2.80 apiece. If Elena purchased a total of 10 of these pens for $42.00, how many brand X pens did she purchase?	Choose the correct choice from the following answers:	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=5 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: 5.
<result>B</result>
Elena purchased Brand X pens for $4.00 apiece and brand Y for $2.80 apiece. If Elena purchased a total of 10 of these pens for $42.00, how many brand X pens did she purchase?