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This can be illustrated by a few examples as below.

Here we cannot find the cost of 5 pens unless we find the cost of one pen, which is also not known, from the question.

We start by assuming the cost of one pen to be ‘x’,

So the cost of 12 pens would be 12 (x) = 12x……………….. (1)

But according to the question cost of 12 pens = $50….…. (2)

So from the equation (1) and (2) above, we get

12x = 50

Dividing both sides by 12

12x/12 = 50

x = 4.16, i.e. the cost of one pen………… (3)

Now, cost of 5 pens would be 5x. But x = 4.16 as per equation (3)

So, cost of 5 pens = 5 X 4.16 = $20.8

Solution:

Let us assume the cost price (C.P) to be =x

We know that,

Selling price (S.P) = C.P + Profit

Or S.P = C.P + P

Or, S.P = x + 20% of x (because profit is 20% of the C.P)

Or, S.P = x + 20x/100

Or, S.P. =120x/100…………………………………..(1)

But according to the question S.P = 200……….(2)

From equation (1) and (2) above

= 200

Multiplying both sides by 100

Or, 120x/100 * 100 = 200 x 100

Or,120x = 20000

Dividing both sides by 120

120x/120 = 20000/120

X= 166.67, or the cost price of the bag was $166.66