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diff --git a/1271/CH13/EX13.20/20.txt b/1271/CH13/EX13.20/20.txt
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+++ b/1271/CH13/EX13.20/20.txt
@@ -0,0 +1,5 @@
+
+ Inter planner distances are - 
+ (1) in first case 6.298630 A, 
+ (2) in second case 4.453804 A ,
+ (3) in third case 3.636515 A 
+\ No newline at end of file
diff --git a/1271/CH13/EX13.20/example13_20.sce b/1271/CH13/EX13.20/example13_20.sce
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+clc 
+// Given that
+lambda = 1.24e-10 // wavelength of X-ray in A
+x1 = 1 // coordinate on x axis of first plane
+y1 = 0 // coordinate on y axis of first plane
+z1 = 0 // coordinate on z axis of first plane
+x2 = 1 // coordinate on x axis of second plane
+y2 = 1 // coordinate on y axis of second plane
+z2 = 0 // coordinate on z axis of second plane
+x3 = 1 // coordinate on x axis of third plane
+y3 = 1 // coordinate on y axis of third plane
+z3 = 1 // coordinate on z axis of third plane
+M = 74.5 // molecular weight of KCl
+d = 1980 // density of KCl in kg/m^3
+N = 6.023e+26 // Avogadro's No per Kg mole
+// Sample Problem 20 on page no. 13.31
+printf("\n # PROBLEM 20 # \n")
+printf(" \n Standard formula used are D = 1/sqrt(x^2+y^2+z^2) and a^3 = n*M/(N*d)")
+a = (4*M / (N*d))^(1/3)
+D1 = a/sqrt(x1^2 + y1^2 + z1^2)
+D2 = a/sqrt(x2^2 + y2^2 + z2^2)
+D3 = a/sqrt(x3^2 + y3^2 + z3^2)
+printf("\n Inter planner distances are - \n (1) in first case %f A, \n (2) in second case %f A ,\n (3) in third case %f A",D1*10^10,D2*10^10,D3*10^10)