A bucket is in the form of a frustum of a cone.

Greater diameter of the frustum  = 56 cm
Greater radius of the frustum = R = 28 cm
Smaller diameter of the frustum = 42 cm
Radius of the smaller end of the frustum = r = 21 cm
Height of the frustum = h = 15 cm

Capacity of the frustum

$=\frac{1}{3} \pi h\left(R^{2}+r^{2}+R r\right)$

$=\frac{1}{3} \times \frac{22}{7} \times 15\left(28^{2}+21^{2}+28 \times 21\right)$

$=\frac{22 \times 5}{7} \times 1813$

$=28490 \mathrm{~cm}^{3}=28.49$ litres