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You look at the second line on the sign and go to the bins and get 1 apple and 3 oranges and put Rs.15 beside them on the counter. You repeat this 4 times and now you have 4 apples and 12 oranges with Rs. 60 there for payment. Then you realize that you don't want any apples so, looking at the first line of the sign you put 4 apples and 2 oranges back in the bins and remove Rs. 30 from the payment. What remains on the counter is 10 oranges and Rs. 30. Now it is easy to see that each orange costs Rs. 3. Looking again at the second line on the sign, 3 oranges cost $3 \times 3 = \mbox{ Rs. }9$ and since $15  9 = 6,$ each apple costs Rs. 6. Algebra: Suppose each apple costs Rs. $x$ and each orange costs Rs. $y.$ The fact that four apples and two oranges cost Rs. 30, and one apple and 3 oranges costs Rs.15 can then be written as
Multiplying the second equation by $4$ gives the equations
Now subtract the first equation from the second equation to get
and dividing both sides by $10$ yields
Substitute $y = 3$ into the equation $x + 3y = 15$ to obtain $x + 3 \times 3 = 15$ and hence $x = 6.$ I hope this helps,  


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