diff --git a/mba/ch40.org b/mba/ch40.org
index c1859e7..46b1665 100644
--- a/mba/ch40.org
+++ b/mba/ch40.org
@@ -46,4 +46,9 @@ How do they forcast
 if we put $1 into inventory it won't be converted into cash for another 50 days
 
 ** Inventory to sale conversion period
-\(\text{ISCP} = \frac{\text{average inventories}}{\text{COGS} \cdot 365}\)
+- you can also do this by season, just replace 365 days with 90 days
+\(\text{ISCP} = \frac{\text{average inventories}}{\text{COGS} \times 365}\)
+
+** Purchase to Payment Conversion Period
+- pay your bills as late as you can
+\(\text{Purchase to Payment Conversion Period} = \frac{\text{average payables} + \text{average accrued liabilities}}{\text{COGS} \times 365}\)