CodingBat

Math and conditionals

 * http://codingbat.com/prob/p238805
 * http://codingbat.com/prob/p283727
 * http://codingbat.com/prob/p283759
 * http://codingbat.com/prob/p267977
 * http://codingbat.com/prob/p232742
 * http://codingbat.com/prob/p208025

Strings

 * http://codingbat.com/prob/p253286
 * http://codingbat.com/prob/p229309
 * http://codingbat.com/prob/p220147

A bit surrpised it seems to simple and yet useful.