Chapter 9 - Section 9.7 - Common Logarithms, Natural Logarithms, and Change of Base - Exercise Set - Page 585: 27

-4

A common logarithm is a logarithm to base 10. As shown on page 580, when a common logarithm is written in the form $log (x)$, this is equivalent to the expression $log_{10}x$. Therefore, $log .0001=log_{10}.0001=-4$. We know this, because $10^{-4}=\frac{1}{10^{4}}=\frac{1}{10000}=.0001$.