Question 1184398
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Knowing how to set up and solve the problem using formal algebra is useful.  Presumably the question is from a student learning algebra; so you should know how to do it, as shown by the other tutors.<br>
But you can get some good mental exercise by solving the problem using logical analysis and some easy mental calculations.<br>
2/7 of his money will buy 3 mechanical pencils; that would leave him with 5/7 of his money.<br>
When he buys 6 mechanical pencils and 3 pens with the remaining 5/7 of his money, the 6 mechanical pencils cost twice as much as 3 of them, which is 2*(2/7)=4/7 of his money.<br>
That means 5/7-4/7=1/7 of his money is used to buy the 3 pens.<br>
And if 1/7 of his money will buy 3 pens, spending all his money on pens only will buy 3*7=21 pens.<br>