Let the cost price of the second book be \( x \) rupees. Then, the cost price of the first book will be \( x - 10 \) rupees.
According to the given information, the total cost price of the two books is Rs. 180. Therefore, we can set up the following equation:
\[ x + (x - 10) = 180 \]
Simplifying the equation:
\[ 2x - 10 = 180 \]
Adding 10 to both sides:
\[ 2x = 190 \]
Dividing by 2:
\[ x = 95 \]
Now that we have the cost price of the second book, we can find the cost price of the first book:
\[ \text{Cost price of first book} = x - 10 = 95 - 10 = 85 \]
Thus, the cost price of the first book is Rs. 85.
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