There is one number that makes investors walk away from some very good businesses: Net profit margin. A company earns only Rs. 3, Rs. 4 or Rs. 5 on every Rs. 100 of sales, and the conclusion comes quickly. The business looks fragile. There appears to be little room for error. A small rise in cost, a weak customer, or one bad year could push profits sharply lower.That instinct is not entirely wrong. But it is incomplete. A low margin tells you