Need for Speed  Subtraction: 0 to 9  
Common Core Math Alignment: 2.OA.2. Fluently add and subtract within 20 using mental strategies.
This work is licensed under a Creative Commons AttributionNonCommercialNoDerivs 3.0 Unported License. 

50% (250 x 300) 
75% (375 x 450) 
100% (500 x 600) 
150% (750 x 900) 
200% (1000 x 1200) 
300% (1500 x 1800) 