Water percent

A 15 g sample of the substance contains 70% water. After drying, the substance reduced its weight to 9 g. What percentage of water is now in the sample?

Correct answer:

p =  50 %

Step-by-step explanation:

$$\begin{gathered} m_1=15\text{g} \\ q=70\%=\frac{70}{100}=\frac{7}{10}=0.7 \\ {m}_2=9\text{g} \\ v=q\cdot m_1=\frac{7}{10}\cdot 15=\frac{7\cdot 15}{10}=\frac{105}{10}=\frac{21}{2}=10.5\text{g} \\ s=m_1-v=15-\frac{21}{2}=\frac{15\cdot 2}{2}-\frac{21}{2}=\frac{30}{2}-\frac{21}{2}=\frac{30-21}{2}=\frac{9}{2}=4.5 \\ p=100\cdot \frac{m_2-s}{m_2}=100\cdot \frac{9-\frac{9}{2}}{9}=50\% \end{gathered}$$

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