To find out how much baking soda Lisa should add to 9 lbs. of flour, we can set up a proportion based on the ratio of baking soda to flour.
Lisa uses 2 tsp of baking soda for 6 lbs. of flour.
First, we can find the amount of baking soda needed per pound of flour: \[ \text{Baking soda per lb} = \frac{2 \text{ tsp}}{6 \text{ lbs}} = \frac{1}{3} \text{ tsp/lb} \]
Next, we can calculate how much baking soda is needed for 9 lbs. of flour: \[ \text{Baking soda for 9 lbs} = 9 \text{ lbs} \times \frac{1}{3} \text{ tsp/lb} = 3 \text{ tsp} \]
Thus, Lisa should add 3 tsp of baking soda to 9 lbs. of flour. The correct response is:
3 tsp.