[ Arithmetic. Mental calculation, etc.]

It is known that 9 cups of tea cost less than 10 rubles, and 10 cups of tea cost more than 11 rubles. How much does a cup of tea cost

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Write the condition in the form of inequalities and use the fact that the cost of a glass of tea is expressed as an integer number of kopecks.

## Solution

Let the cost of a glass of tea, expressed in kopecks, be \( x \). Then, according to the condition, \( 9x \leq 1100 \).

Therefore, \( 9x \) does not exceed 999, and \( x \) does not exceed 111. \( x = 111 \) fits - in this case, \( 10x = 1110 > 1100 \). If, however, \( x < 111 \), then \( x \) does not exceed 110, and in this case, \( 10x \) does not exceed 1100. Thus, there is only one possibility: \( x = 111 \).

## Answer

1 ruble 11 kopecks