If the quantity demanded of rice de- creased from 150 bags to 100 bags, the percentage change in quantity demanded is

Analyze the question The question asks for the percentage change in quantity demanded when it falls from 150 bags to 100 bags.

Concept Percentage change = (change in quantity ÷ original quantity) × 100

The original quantity is always the base, so we divide by the starting value (150), not the new one.

Solution

1. Change in quantity = 100 − 150 = −50 bags
2. Original quantity = 150 bags
3. Percentage change = (−50 ÷ 150) × 100
4. = −0.3333 × 100
5. = −33.3%

The negative sign shows a decrease, so the quantity demanded fell by 33.3%.

Checking the other options

• A (30.0%) is a rough guess and does not match the calculation.
• C (50.0%) comes from wrongly dividing by the new quantity (50 ÷ 100) or from using the absolute change.
• D (66.7%) comes from dividing by the wrong base, such as 100 ÷ 150.

Answer: B (33.3%)

Exam tip Always divide by the original (starting) value. A quick check: 150 → 100 means losing one-third of the bags, and one-third is 33.3%. Also remember that in percentage change questions, a decrease is shown with a negative sign, but the options often give just the size of the change.