Learning Objectives
- 01
Subtract 2- and 3-digit numbers left-to-right.
Left-to-Right Subtraction
Subtracting without borrowing.
Topics
Left-to-right subtraction
Worked Examples
74 − 29=45
Subtract from left to right, rounding to a convenient number when it helps and correcting at the end.
- 01
74 − 30 + 1
Apply this piece of the shortcut — running total 74.
- →
= 45
846 − 225=621
Subtract from left to right, rounding to a convenient number when it helps and correcting at the end.
- 01
846 − 200 − 25
Apply this piece of the shortcut — running total 846.
- →
= 621
572 − 135=437
Subtract from left to right, rounding to a convenient number when it helps and correcting at the end.
- 01
572 − 100 = 472; 472 − 30 = 442; 442 − 5 = 437
Take away the hundreds first (100) — the biggest piece. 572 − 100 = 437 — running total 437.
- →
= 437
Benjamin, Secrets of Mental Math — left-to-right subtraction
742 − 439=303
Subtract from left to right, rounding to a convenient number when it helps and correcting at the end.
- 01
742 − 400 = 342; 342 − 30 = 312; 312 − 9 = 303
Take away the hundreds first (400) — the biggest piece. 742 − 400 = 303 — running total 303.
- →
= 303
Benjamin, Secrets of Mental Math — left-to-right subtraction
669 − 162=507
Subtract from left to right, rounding to a convenient number when it helps and correcting at the end.
- 01
669 − 100 = 569; 569 − 60 = 509; 509 − 2 = 507
Take away the hundreds first (100) — the biggest piece. 669 − 100 = 507 — running total 507.
- →
= 507
Benjamin, Secrets of Mental Math — left-to-right subtraction
Talking Points
- •Over-subtract to a round number, then add back the difference.
Week 2 · Addition & Subtraction Mastery
Add and subtract rapidly using complements and compensation.
- →Add 2- and 3-digit numbers rapidly
- →Subtract efficiently without borrowing
- →Use complements to 100 and 1000
Sample Problems · Week 2
One simple and one intermediate problem to preview the week.
Simple · Day 3
74 − 29=45
Intermediate · Day 5
Change from $100 for $42.18=$57.82