1. Let's start with a simple fractions aptitude test problem: Simplify the fraction $\frac{18}{24}$.\n\n2. The formula to simplify a fraction is to divide both the numerator and the denominator by their greatest common divisor (GCD).\n\n3. Find the GCD of 18 and 24. The factors of 18 are 1, 2, 3, 6, 9, 18. The factors of 24 are 1, 2, 3, 4, 6, 8, 12, 24. The greatest common factor is 6.\n\n4. Divide both numerator and denominator by 6: $$\frac{18 \div 6}{24 \div 6} = \frac{3}{4}.$$\n\n5. So, the simplified form of $\frac{18}{24}$ is $\frac{3}{4}$.\n\nThis test problem helps you practice finding the GCD and simplifying fractions.