The thickness of a layer of ice in a water body is $0.00205\ \text{m}$.

1. Rewrite the number with a coefficient between 1 and 10 and choose the correct power of 10.

$0.00205=2.05\times10^{-3}$

The decimal point moves 3 places to the right to get $2.05$, so the power is $-3$.

1. Calculate the volume of ice/water in cubic metres.

Thickness $=0.00205\text{ m}=2.05\times10^{-3}\text{ m}$

$\text{Volume}=\text{area}\times\text{thickness}$

$=(1.60\times10^5)(2.05\times10^{-3})=328\text{ m}^3$

2. Convert the volume from cubic metres to litres.

Since $1\text{ m}^3=1000\text{ litres}$,

$328\text{ m}^3=328\times1000=328000\text{ litres}$

1. Calculate the volume of rainwater added in two hours.

$2\text{ h}=2\times60\times60=7200\text{ s}$

Rainwater added
$=5\times10^{-1}\times7200=0.5\times7200=3600\text{ l}$

2. Express the increase as a percentage of the original volume of water.

Original volume from part (b) $=410\text{ l}$.

Percentage increase
$=\frac{3600}{410}\times100\%=878.048\ldots\%$
$\approx878\%$