small edits

Author: jittat

BNK-rand.ipynb

@@ -154,6 +154,8 @@
     "$$E[Y] = E\\left[\\sum_{i=1}^{N} Y_i\\right] = \\sum_{i=1}^N E[Y_i] = N\\cdot Pr[B]\n",
     "= N\\left(\\sum_{P=0}^{L} q(P)\\cdot {L \\choose P}(M/N)^P(1-M/N)^{L-P}\\right).$$\n",
     "\n",
+    "Note that the second step holds because of the linearity of expectation.\n",
+    "\n",
     "#### Let's see the numbers\n",
     "\n",
     "We will plug in, again, $N=27$ and $M=5$."