Answer: 
She is not correct. The correct price is $0. 50
Step-by-step explanation:
The unit price is found by dividing the total price by the number of drinks 
Now, in our case 
Total price = $2.99 
and 
Total number of drinks = 6 
Therefore, 
unit price = $2.99/6 = $0. 50 (rounded to the nearest tenth ) 
Hence, we see that the unit price that Devon's sister mentioned cannot be correct since the actual price is about $0.50.