When water is changed into ice, its volume increases by 9%. If ice change into water, the percentage decrease in volume is :

Let V denote the volume of 1 c.c. of water
Then, volume of ice formed from it :
= 109% of V = $$\frac{109}{100}$$V
When this ice change into water, decrease in volume :
= $$\frac{109}{100}$$V - V = $$\frac{9}{100}$$V
∴ Decrease % :
$$\eqalign{ & = \left( {\frac{{9V}}{{100}} \times \frac{{100}}{{109V}} \times 100} \right)\% \cr & = \frac{{900}}{{109}}\% \cr & = 8\frac{{28}}{{109}}\% \cr} $$