The price of 3 notebooks and 6 pen is Rs. 3000. With the same money one can buy a notebooks and 12 pen. Raju wants to buy 15 pen, how much will he have to pay?

Given:​
The Price of  3 Notebook  + 6 Pen = Rs. 3000
The Price of one Notebook + 12 Pen = Rs. 3000
Solution: 

Let notebook price be N and pen price be P.
3N + 6P = 3000
N + 12P = 3000
3(3000−12P) + 6P = 3000
9000 − 36P + 6P = 3000
9000 − 30P = 3000
−30P = 3000 − 9000
−30P = −6000
P = −6000/-30​
P = 200
Cost of 15 pens = 15 × 200 = Rs. 3000​​