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diff --git a/chapter-1/ex-21.py b/chapter-1/ex-21.py
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+#!/usr/bin/env python
+
+import matplotlib.pyplot as plt
+import numpy as np
+import random
+
+p = 0.3
+n = 1000
+
+def experiment(p, n):
+    proportion = np.empty(1000)
+    heads = 0
+    random.seed()
+
+    for i in range(n):
+        if random.random() < p:
+            heads += 1
+        proportion[i] = heads/(i+1)
+
+    f1 = plt.figure(1)
+    return proportion
+
+xaxis = np.arange(1,1001)
+tosses = np.empty(1000)
+proportion1 = experiment(0.3, 1000)
+proportion2 = experiment(0.03, 1000)
+
+f1 = plt.figure(1)
+plt.plot(xaxis, proportion1)
+
+f2 = plt.figure(2)
+plt.plot(xaxis, proportion2)
+
+plt.show()