12 - 3 = 9
3 + 9 = 12
12 - 9 = 3
Here is the equation:

Since both decimals are negative, the answer will be positive.
- Negative times Negative = Positive
Divide both sides by -3.9 to leave the variable alone:

r is equal to 4 → r = 4
Answer:
2.9 litres
or
2 14/15 litres
Step-by-step explanation:
To determine the amount of water Sam drinks in an hour, divide the total amount drunk by the number of hours
Amount drank in an hour = total litres drunk / time
÷ 
×
=
= 2
litres