[tex]Let \: the \: Number \: be \: x \\ \\ x \div \frac{15}{3} = 5 \\ \\ x = 3 \times \frac{3}{15} \\ \\ x = \frac{3}{5} [/tex]

Answer:

Let the other no. be x

15/3 ÷ x = 5

x = 5*3/15

x = 3/15