1. The problem asks to find \textit{one quarter} of 80 marbles. 2. The phrase "one quarter" means \frac{1}{4} of a quantity. 3. The formula to find a fraction of a number is: $$\text{fraction} \times \text{whole number}$$ 4. Applying this to our problem: $$\frac{1}{4} \times 80$$ 5. Multiply numerator and denominator: $$\frac{1 \times 80}{4} = \frac{80}{4}$$ 6. Simplify the division: $$80 \div 4 = 20$$ 7. Therefore, one quarter of 80 marbles is 20 marbles. This means if you divide 80 marbles into 4 equal parts, each part has 20 marbles.