Algebra

 

Laverne starts counting out loud by 5's.  She starts with 2.  As Laverne counts, Shirley sums the numbers Laverne says.  When the sum finally exceeds 20, Shirley runs screaming from the room.  What number does Laverne say that sends Shirley screaming and running?   

 

Laverne's           Shirley's   

Counting          Summation   

 

     2                           2   

     7                           9    

   12                         21    

_.