clc;
clear;
D_4=0.4 //diameter of the 4th dark ring in cm
D_12=0.7 //diameter of the 12th dark ring in cm

//calculation
D_20=sqrt(2*((0.7^2)-(0.4^2))+0.4^2)
mprintf("The diameter of the 20th dark ring is = %1.3f cm",D_20)