I moved the original post down to my second post and made a quick index of everything I've done here so it's easier to see what's going on.

Memorizing Fractions: Halves up to Tenths
http://www.flopturnriver.com/phpBB2/...10.html#702810

Approximating Square Roots Mentally (second half of post)
http://www.flopturnriver.com/phpBB2/...10.html#702810

Approximating Base 10 Logarithms Mentally
http://www.flopturnriver.com/phpBB2/...24.html#703624

Approximating Sine Mentally
http://www.flopturnriver.com/phpBB2/...35.html#703635

Using Sine Approximations to Find Cosine, Tangent, Cotangent, Secant, and Cosecant
http://www.flopturnriver.com/phpBB2/...36.html#703636

Divisibility Tests for 2, 3, 5, 6, 9
http://www.flopturnriver.com/phpBB2/...42.html#703642

Divisibility Tests for 4, 8
http://www.flopturnriver.com/phpBB2/...46.html#703646

Divisibility Test for 7
http://www.flopturnriver.com/phpBB2/...12.html#703712

Multiplying Two Two-Digit Numbers Mentally
http://www.flopturnriver.com/phpBB2/...16.html#703716

Figuring a 15% Tip Mentally
http://www.flopturnriver.com/phpBB2/...56.html#704956

Squaring Numbers Quickly
http://www.flopturnriver.com/phpBB2/...08.html#705008