Problem 4: (first taught in lesson 59) Write the fraction $\frac{3}{15}$ as a ratio with a colon.

Step 1 ：Simplify the fraction \(\frac{3}{15}\) by dividing both the numerator and the denominator by their greatest common divisor (3): \(\frac{3}{15} = \frac{3 \div 3}{15 \div 3} = \frac{1}{5}\)

Step 2 ：Replace the division symbol with a colon: \(\frac{1}{5} \rightarrow 1:5\)

Step 3 ：\(\boxed{1:5}\) is the final answer.