Express the following ratios as (A) fraction (B) Decimal (C) percentage

$$
\frac{3}{5} = \frac{15}{X}
$$

$$
3X = 5 \times 15
$$
$$
3X = 75
$$

$$
X = \frac{75}{3}
$$
$$
X = \boxed{25}
$$

$$
\frac{2}{7} = \frac{Y}{56}
$$

$$
7Y = 2 \times 56
$$
$$
7Y = 112
$$

$$
Y = \frac{112}{7}
$$
$$
Y = \boxed{16}
$$

$$
\text{Cost of Pen} : \text{Cost of Pencil} = 15:25 = 3:5
$$

$$
\text{Total Cost} = 15k + 15k = \boxed{30k}
$$
(Note: The question does not provide a specific value for the total cost, so the answer is expressed in terms of $k$.)

$$
\frac{\text{Cost of Pen}}{\text{Cost of Pencil}} = \frac{3}{5}
$$

$$
\frac{3}{5} \times 100\% = 0.6 \times 100\% = \boxed{60\%}
$$