When water becomes ice, its volume increases by $$10\%$$. When ice melts into water its volume decreases by $$a\%$$. Then $$a$$ =

If the volume of water is $$x$$, the volume of ice is $$x + \frac{10}{100}x = \frac{11}{10}x$$. Melting back, the decrease from ice volume to water volume is $$\frac{11}{10}x - x = \frac{1}{10}x$$, which as a percentage of the ice volume is $$\frac{\frac{1}{10}x}{\frac{11}{10}x} \times 100 = \frac{100}{11} \approx 9.09$$. So $$a \approx 9.09$$.