Lesson video
Start here.
Press play for Tuesday’s lesson on adding and subtracting to solve. Then catch the Bandit Error further down the page and finish with the Mission.
October · Week 1 · Day 2
Add and Subtract to Solve
Adding is undone by subtracting, and subtracting is undone by adding. Today you pick the right undo key — and find out that a perfectly legal move can land below zero.
Three things to lock in.
These are the ideas the Mission asks you to use.
Inverse operationThe opposite move that undoes an operation. + 12 is undone by − 12.
Same move, both sidesSubtract 12 from the left and from the right, on the same line.
Negative answersNothing has gone wrong. 5 − 12 = −7.
✗ Today’s Bandit Error
5 − 12 is not 7.
Bandit gets the move right, then reads the subtraction backwards.
✗ x = 7
✓ x = −7
The sign travels with the answer. 5 − 12 crosses zero, so the solution is negative — and substituting it back shows that immediately.
Free Mission
Ready to try it yourself?
Tuesday’s Mission has 10 problems plus the Mission Debrief so you can check your reasoning when you finish.
No sign-up required.