The one plus one / Jojo Moyes.

Suppose your life sucks. A lot. Your husband has done a vanishing act, your teenage stepson is being bullied, and your math whiz daughter has a once-in-a-lifetime opportunity that you can't afford to pay for. That's Jess's life in a nutshell-until an unexpected knight in shining armor offers to rescue them. Only Jess's knight turns out to be Geeky Ed, the obnoxious tech millionaire whose vacation home she happens to clean. But Ed has big problems of his own, and driving the dysfunctional family to the Math Olympiad feels like his first unselfish act in ages. Maybe ever.