3_b_3: Add solution

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diff --git a/ch03/3_b_3.hs b/ch03/3_b_3.hs
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+-- Write a function that computes the mean of a list, i.e. the sum of all
+-- elements in the list divided by its length. (You may need to use the
+-- fromIntegral function to convert the length of the list from an integer into
+-- a floating point number.)
+
+sumOfList [] = 0
+sumOfList (x:xs) = x + (sumOfList xs)
+
+meanOfList [] = 0
+meanOfList xs = sum / (fromIntegral len)
+  where sum = sumOfList xs
+        len = length xs
+
+-- ghci> :l 3_b_3.hs
+-- [1 of 1] Compiling Main             ( 3_b_3.hs, interpreted )
+-- Ok, one module loaded.
+-- ghci> meanOfList []
+-- 0.0
+-- ghci> meanOfList [1]
+-- 1.0
+-- ghci> meanOfList [1, 3]
+-- 2.0
+-- ghci> meanOfList [2.3, 4.5, 6.9]
+-- 4.566666666666666