Have to start somewhere: Kenken

I have in the last decade tutored college grads, or those soon to be, who were preparing for the GMAT, the test to get into graduate business programs. These people, many of whom had gone to fairly prestigious schools as undergrads, were often unable to factor numbers, multiply single digits in their heads, or reduce fractions without moving their lips and staring off into space. Kenken is quite an entrancing game that can strengthen all those skills, and logic as well. It is available daily in the New York Times online (but you will have to limit your visits to 20 a month or pay a subscription), kenken.com for free (recommended), and there is a decent app for iDevices (fairly inexpensive.)