Master the Assumption Method

Stop using “Guess and Check”! The Assumption Method (or “Supposition Method”) is the faster, smarter way to solve “Total Items + Total Value” problems.

The Calculator

Enter your problem details below. You can rename “Type A” and “Type B” to match your question (e.g., Chickens, Rabbits, Cars, Motorcycles).

Assumption Method Calculator

Total Items (Heads)

Total Value (Legs)

Type A Details

Name

Value (Legs/Points)

Type B Details

Name

Value (Legs/Points)

What is the Assumption Method?

The Assumption Method is a heuristic used to solve problems where you know:

Total Number of Items (e.g., 20 animals)
Total Value (e.g., 50 legs)
Value of Item A (e.g., Chicken has 2 legs)
Value of Item B (e.g., Rabbit has 4 legs)

Instead of guessing, we make a single assumption (e.g., “Assume ALL are Chickens”) and see how far off we are.

The 4-Step Formula

Assume all items are Type A.
Calculate Difference between the Total Value given and your Assumed Value.
Calculate Gap between Type A and Type B.
Divide the Difference by the Gap to find the number of Type B items.

Worked Examples

Example 1: The Classic Chicken & Rabbit

Problem:

A farmer has 10 animals (chickens and rabbits). Together they have 30 legs. How many of each does he have?

Step 1: Assume all are Chickens (2 legs) $10 \times 2 = 20$ legs

Step 2: Find the Difference (Big Gap) Real Total (30) - Assumed Total (20) = 10 legs extra

Step 3: Find the Gap per Animal (Small Gap) Rabbit (4) - Chicken (2) = 2 legs difference

Step 4: Divide $10 \div 2 = 5$ Rabbits

Step 5: Find the Rest $10 - 5 = 5$ Chickens

Check: $(5 \times 4) + (5 \times 2) = 20 + 10 = 30$ legs ✓

Example 2: The Math Test (Marks)

Problem:

A test has 20 questions. 5 marks are awarded for a correct answer, but 2 marks are deducted for a wrong answer. John scored 72 marks. How many questions did he get correct?

⚠️ Watch Out for Deductions!

When marks are deducted, the Gap is bigger! Gap = Correct (5) - Wrong (-2) = 7 marks gap.

Step 1: Assume all Correct $20 \times 5 = 100$ marks

Step 2: Find Difference $100 - 72 = 28$ marks difference

Step 3: Find Gap $5 - (-2) = 7$ marks

Step 4: Divide (Finds Wrong answers) $28 \div 7 = 4$ Wrong

Step 5: Find Correct $20 - 4 = 16$ Correct

When to Use This Method?

Look for these keywords:

“Total number of items”
“Total value/legs/wheels/cost”
Two different types of items
Coins (20-cent and 50-cent)
Tickets (Adult and Child prices)
Vehicles (Cars and Motorcycles)

Practice Assumption Method Questions

Want to test your skills? Try our PSLE Math practice questions on HomeCampus.

Start Practice →

Assumption Method Calculator: Solve "Chicken & Rabbit" Problems Instantly