If a number is divided by D than thr reminder is 8, if the same number is divided

by 3D the reminder is 21, now if twice the number is divided by 3D then what is the reminder?

D = 21 - 8 = 13
then:
21/13 => 1 * 13, R = 8

21/(3 * 13)
= 21/39 = 0 * 39 , R = 21

(2*21)/39
= 42/39 => 1 * 39 , R = 3

Answer => R = 3