How to generate two random irreducible fractions

Use this method when you want students to compute the sum of two fractions and ensure that each generated fraction is already irreducible (i.e., simplified before the operation).

Following this guide, you will generate two random fractions whose numerators and denominators are coprime, guaranteeing that both fractions are irreducible before students calculate their sum.

See it in action: Watch how this logic is implemented inside Nubric:

Before you begin

Requirements

• Basic knowledge of how to create a Nubric question
• Familiarity with adding an algorithm to generate random variables

Steps

Generate the first numerator

a = random(1,9)

This defines the numerator of the first fraction.

Generate a compatible denominator

B = {b with b in 2..9 where gcd(a,b) = 1}
b = random(B)

This creates a set of denominators coprime to a, ensuring the fraction is irreducible.

Generate the second numerator

c = random(1,9)

Generate the second denominator

D = {d with d in 2..9 where gcd(c,d) = 1 && d != b}
d = random(D)

This ensures:

• The second fraction is irreducible.
• The denominators are not identical.

Define the solution

sol = a/b + c/d

This stores the correct answer for grading.

Verify it worked

• Preview the question multiple times.
• Confirm that each generated fraction is already simplified.
• Check that no denominator equals 0.
• Ensure the sum is computed correctly.

Full algorithm (copy-paste version)

Use the complete version below if you want to copy the logic directly into your question algorithm:

# Generate the first fraction
a = random(1,9)    

B = {b with b in 2..9 where gcd(a,b) = 1}  
b = random(B)               

# Generate the second fraction
c = random(1,9)   

D = {d with d in 2..9 where gcd(c,d) = 1 && d != b} 
d = random(D)                  

# Compute the solution
sol = a/b + c/d

Options and variations

• If you want to allow identical denominators, you can remove the condition d != b from the second denominator set.
• If you want larger fractions, you can increase the interval (e.g., 1..15 instead of 1..9), but test performance to avoid excessive regeneration.
• If you want to enforce simplified student answers, you can combine this pattern with answer validation constraints in grading logic.

Common errors

Fractions are not irreducible.
Check that gcd(a,b)=1 is correctly written in the set definition

Infinite regeneration loop.
Ensure the denominator interval contains enough values to satisfy the coprimality condition

Unexpected duplicate fractions.
Add additional constraints if you need both fractions to differ entirely

Related

• Understanding Advanced Logic in Nubric
• Common Patterns and Best Practices
• Glossary of Commands