diff --git a/mathgenerator/funcs/arithmetic_progression_sum.py b/mathgenerator/funcs/arithmetic_progression_sum.py
index d698354..56e2760 100644
--- a/mathgenerator/funcs/arithmetic_progression_sum.py
+++ b/mathgenerator/funcs/arithmetic_progression_sum.py
@@ -5,12 +5,12 @@ def arithmeticProgressionSumFunc(maxd=100, maxa=100, maxn=100):
     d = random.randint(-1 * maxd, maxd)
     a1 = random.randint(-1 * maxa, maxa)
     a2 = a1 + d
-    a3 = a2 + d
+    a3 = a1 + 2*d
     n = random.randint(4, maxn)
     apString = str(a1) + ', ' + str(a2) + ', ' + str(a3) + ' ... '
-    problem = 'Find the sum of first ' + str(
-        n) + ' terms of the AP series: ' + apString
-    solution = n * ((2 * a1) + ((n - 1) * d)) / 2
+    problem = 'Find the sum of first ' + str(n) + ' terms of the AP series: ' + apString
+    an = a1+(n-1)*d
+    solution = n * (a1+an) / 2
     return problem, solution