Black marker in hand, Nick Baar returned to the whiteboard. The newly minted math tutor wrote an equation for the eighth-graders in front of him: 25 x 199 The rules were simple: No calculators or pencils allowed. Baar wanted to see if his students at Perry Street Preparatory, a charter school in Northeast Washington, could solve the problem in their heads. They had breezed through an earlier problem — 25 x 200 — but now appeared stumped. The group sat in silence. One boy spoke up. He...