There are quite a few possible combinations that are minimum 8 sodas, and under $20.
1. 4 Dr. Peppers and 4 Sprites. $2.98 times 4 = $11.92. $1.99 times 4 = $7.96. $7.96 + $11.92 = $19.88.
2. 3 Dr. Peppers and 5 Sprites. $2.98 times 3 = $8.94. $1.99 times 5 = $9.95. $9.95 + $8.94 = $18.89.
3. 2 Dr. Peppers and 6 Sprites. $2.98 times 2 = $5.96. $1.99 times 6 = $11.94. $11.94 + $5.96 = $17.90.
4. 2 Dr. Peppers and 7 Sprites. $2.98 times 2 = $5.96. $1.99 times 7 = $13.93. $5.96 + $13.93 = $19.89.
5. 1 Dr. Pepper and 7 Sprites. $2.98 times 1 = $2.98. $1.99 times 7 = $13.93. $2.98 + $13.93 = $16.91.
6. 1 Dr. Pepper and 8 Sprites. $2.98 times 1 = $2.98. $1.99 times 8 = $15.92. $15.92 + $2.98 = $18.90.
I hope this helps!