The volumes of the substances:
\(V_{water}=10 \ mL \\
V_{alcohol}= 10 \ mL \times 7\%=10 \ mL \times 0.07=0.7 \ mL\)
x - the amount of water that should be added
\(\frac{0.7}{10+x}=5\% \\
\frac{0.7}{10+x}=0.05 \ \ \ \ \ \ \ \ \ \ \ |\times (10+x) \\
0.7=0.05(10+x) \\
0.7=0.5+0.05x \ \ \ \ |-0.5 \\
0.2=0.05x \ \ \ \ \ \ \ \ \ \ \ |\times 20 \\
x=4\)
4 mL of water should be added.
