Use the units method: blue paint = 2 units, red paint = 3 units.<br><br>
[[model:questions/solutions/q7226.png]]<br><br>
&bull; 2 units → 10 L of blue paint, so 1 unit → 10 ÷ 2 = 5 L<br>
&bull; Red paint = 3 units → 3 × 5 = 15 L<br><br>
You need 15 litres of red paint.<br>
Check: Blue : Red = 10 : 15 = 2 : 3. ✓