Store stops

Karel always follows the same route to school. If he continues at the store, the journey takes him exactly 10 minutes. If he stops at a store to buy a snack, his journey takes 5 minutes longer. In November 2015, his journey to school took an average of 13 minutes. How many times did he stop at the store that month?

Final Answer:

x =  12

Step-by-step explanation:

$$\begin{gathered} n=20 \\ (10n+5x)/n=13 \\ 10+5x/n=13 \\ x=(13\cdot n-10\cdot n)/5=(13\cdot 20-10\cdot 20)/5=12 \end{gathered}$$

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