The cost of 3 pens and 5 pencils is Rs.240. Also the cost of one pen and one pencil is in the ratio of 5:1 respectively. What is the cost of one dozen pens? Choose the correct choice.
A) Rs.200
B) Rs.720
C) Rs.300
D) Rs.150
E) None of these
Explanation:   Let the cost of one pen is ‘5x’ and pencil is ‘x’   3 X 5x + 5x = Rs.240   15x + 5x = Rs.240   X = 240/20= 
<gadget id="calculator">240/20</gadget>
<output>12</output>
12   :. Cost of 1 pen= 5x = 5 x 12 = 60   :. Cost of 12 pens, i.e. (one dozen) = 60 x 12 = Rs.720   Answer: Option Rs.720
<result>B</result>
The cost of 3 pens and 5 pencils is Rs.240. Also the cost of one pen and one pencil is in the ratio of 5:1 respectively. What is the cost of one dozen pens?