You can earn extra income using this free Android app from Play Store. Try Math Rewards - Solve & Earn Money - Apps on Google Play.
In this app, you will compete with other real players worldwide and you will be given 20 sets of basic Math expressions (e.g. 4 + 5, 3 + 14) to solve within 1 minute. Whoever is the fastest to solve and gets the highest score will be the winner.
There are also many ways to earn points aside from solving Math. Points accumulated can be converted to real cash thru Paypal once you reach the minimum threshold. For more details about this app, kindly check this out: Earn Money by Solving Basic Math Problems using Your Phone