Hi Trina,

You have 2500 mL of an fluoride/water solution that is 0.91% acid and thus you have

2500 × 0.0091 mL of fluoride and
2500 × 0.9909 mL of water.

You want to add x mL of 49% acid/water solution. These x mL are composed of

x × 0.49 mL of fluoride and
x × 0.51 mL of water.

Hence in total you have

2500 × 0.0091 + x × 0.49 mL of fluoride (call this amount P) and
2500 × 0.9909 +x × 0.51 mL of water.

The total volume of liquid is (2500 × 0.0091 + x × 0.49) + (2500 × 0.9909 +x × 0.51) mL (call this amount Q).

The goal is for P to be 1.40% of Q. Solve for x.

Harley