" This is an extreme example, but easy math. If a $20.00 stock declines by 50% to $10.00, you will need a 100% gain to break even. Why? 100 shares of a $20.00 stock are worth $2,000.00 dollars. After a 50% decline, your investment is now only $1,000.00. So, even if the stock rebounds 50% back to the original price on the following day, your $1,000.00 is now only $1,500.00. 75% down, you need 300% to get back to even. If you were wrong on a stock. Take a loss. It's tuition. Use what you have left to make better choices. "

― John Endris , Smart Money: Ten Proven Strategies that Will Increase Your Stock Market Returns