Mary

Mary is 12 years old, which is 40% of the age of her mother. How many percent of her mother's age will Mary have after the next 12 years?

Correct result:

p =  57.1429 %

Solution:

$$\begin{gathered} 12=0.40\cdot m \\ 0.4m=12 \\ m=30 \\ p=100\cdot {\displaystyle \frac{12+12}{m+12}}=100\cdot {\displaystyle \frac{12+12}{30+12}}={\displaystyle \frac{400}{7}}=57.1429\mathrm{\%} \end{gathered}$$

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