To find out how much sugar Sebastian needs for the lemon bars, we can multiply the amount of sugar needed for the cookies by 2.5.
The recipe for gingerbread cookies requires 1.5 cups of sugar. Therefore, the amount of sugar needed for the lemon bars is:
\[ 1.5 \text{ cups} \times 2.5 = 3.75 \text{ cups} \]
So, Sebastian needs 3.75 cups of sugar for the lemon bars.