To calculate $41^2$, David mentally figures the value $40^2$ and adds 81. David subtracts a number from $40^2$ to calculate $39^2$. What number does he subtract?
39^2 = (40 -1) (40 -1) = 40^2 - 2(40) + 1 = 1600 - 79
He subtracts 79
+82 
