You must use 2.50 mL of the concentrated solution to make 10.0 mL of the dilute solution.
We can use the dilution formula
<em>V</em>_1<em>C</em>_1 = <em>V</em>_2<em>C</em>_2
where
<em>V</em> represents the volumes and
<em>C</em> represents the concentrations
We can rearrange the formula to get
<em>V</em>_2 = <em>V</em>_1 × (<em>C</em>_1/<em>C</em>_2)
<em>V</em>_1 = 10.0 mL; <em>C</em>_1 = 5.00 g/100. mL
<em>V</em>_2 = ?; ____<em>C</em>_2 = 20.0 g/100. mL
∴ <em>V</em>_2 = 10.0 mL × [(5.00 g/100. mL)/(20.0 g/100. mL)] = 10.0 mL × 0.250
= 2.50 mL