Ratios, Rates, Proportional Relationships, and Units

Currently $24:36 = 2:3$. To make red:blue $= 1:2$, red would need to decrease, not increase. The current ratio is $2:3$ which means red is $2/5$. For ratio $1:2$, red is $1/3$. Since adding red increases red proportion further from $1:3$, we'd need $0$ added if interpretation is no addition, but this is a trick. Actual: ratio $1:2$ means red:blue. With $24$ red and $36$ blue, we have $24/36 = 2/3$, equivalent to $4:6$ not $1:2$. To get $1:2$, we'd reduce red to $18$. So adding zero is consistent with the closest answer if no removal allowed.