#### Answer

$3.0$ mL. The precision of the measurement of the first sample is the limit.

#### Work Step by Step

The first cylinder contains 3.0 mL of water (This is actually between 2.9 and 3.1 because of the precision of the measurement. )
The second cylinder contains 0.30 mL of water (between 0.29 and 0.31).
The sum of those two is $3.0+0.30=3.30$ mL of water.
However, in addition, the result has the same number of decimal places as the least precise measurement used. So, the result should be written as $ 3.3 $ mL as $3.0$ has two significant digits.