Complete Question:
If a 100kg bag of rice cost #120, how many kilogram of rice can one buy for #40 
Answer:
33 kilograms. 
Explanation:
Given the following data;
100 kg of rice = #120
1 kg of rice = #x
Cross-multiplying, we have;
100x = 120
x = 120/100
x = 1.2
Now, to find how much #40 can buy;
 
Substituting into the equation, we have;
 
Quantity of rice = 33.33 ≈ 33 kilograms. 
Method II
100 kg of rice = #120
x kg of rice = #40
Cross-multiplying, we have;
120x = 40 * 100
x = 4000/120
x = 33.33 ≈ 33 kilograms.