Two parallel chords are drawn in a circle of diameter 30 cm. The length of one chord is 24 cm and the distance between the two chords is 21 cm. The length of the other chord is

AB = 24 cm

AE = EB = 12 cm

$\mathrm{OE}=\sqrt{\mathrm{OA}^{2}-\mathrm{AE}^{2}}=\sqrt{15^{2}-12^{2}}$

$=\sqrt{225-144}=\sqrt{81}=9 \mathrm{~cm}$

$\therefore \quad$ OF $=21-9=12 \mathrm{~cm}$

$\therefore \quad C F=\sqrt{15^{2}-12^{2}}=9 \mathrm{~cm}$

$\therefore \quad C D=2 \times 9=18 \mathrm{~cm}$