Predict the result
Subtract positive twenty from negative four.

Set up the model
Subtraction can be rewritten as addition of the opposite.
Change the operation and the sign of the quantity being subtracted.

Work the example
Negative four minus twenty is negative four plus negative twenty.
The result is negative twenty four.

Check the result
Now subtract negative six from negative nine.
Take eight seconds to think before the answer appears.
[Eight-second silent pause to work; the answer remains hidden.]
Add positive six; the result is negative three.

Interpret the result
Subtracting a negative increases the original value.