To find the area of the entire farm, we need to use its dimensions, which are given as 50 cm by 20 cm.
The expression for the area of the entire farm is: \[ \text{Area of the entire farm} = 50 , \text{cm} \times 20 , \text{cm} \]
So, the completed expression is: \[ \text{Area of the entire farm} = 50 , \text{cm} \cdot 20 , \text{cm} \]