15. 3/8 : 1/2 = 2:x

Решение:

1. \[ \frac{3}{8} : \frac{1}{2} = 2 : x \]
2. \[ \frac{3}{8} \times \frac{2}{1} = 2 : x \]
3. \[ \frac{6}{8} = 2 : x \]
4. \[ \frac{3}{4} = 2 : x \]
5. \[ 3x = 4 \times 2 \]
6. \[ 3x = 8 \]
7. \[ x = \frac{8}{3} \]

Ответ: x = 8/3