Added some solutions in python and c

solutions/c/multiples-of-3-and-5.c

@@ -0,0 +1,13 @@
+#include <stdio.h>
+
+int sum_of_multiples(int n)
+{	int sum=0;  
+	n=n-1;
+    sum=((n/3)*((n/3)*3 +3))/2 + ((n/5)*((n/5)*5 +5))/2 - ((n/15)*((n/15)*15 + 15))/2; // sum of airthmetic progressions & De-Morgan's Law
+    return sum;
+}
+int main()
+{
+	printf("%d\n",sum_of_multiples(1000));
+ 	return 0;
+}

solutions/python/sum-of-array-plus-one.py

@@ -0,0 +1,3 @@
+def plusOneSum(arr):
+	"""returns the sum of the integers after adding 1 to each element"""
+	return sum(arr)+len(arr)