Yahoo, understandably, uses American usage (short scale).

According to the short scale, a billion is 10^9, and a trillion is 10^12, thus a billion trillion should be 10^21 (i.e. 1 with 21 zeroes after is), which would otherwise be known as a sextillion.

The old British usage is also known as the long scale.

According to the long scale, a billion is 10^12, and a trillion is 10^18, so a billion trillion would be 10^30 (i.e. 1 with 30 zeroes after it), which would otherwise be known as a quintillion.

http://www.unc.edu/~rowlett/units/large.html

**G***eorge*