2-min Fluency Warm-up — Mental math.
400 − 100 =
250 + 100 =
600 − 100 =
150 + 100 =
Problem Solving Strategy #2: Work Backward
When you know the answer but not the starting point, work backward!
Problem: I had some marbles. I found 18 more. Now I have 52. How many did I start with?
Work backward: 52 − 18 = 34
Answer: I started with 34 marbles
Problem: Sarah read some pages. Then she read 25 more. Now she has read 67 pages total. How many did she start with?
Work backward: 67 − 25 = 42
Answer: She started with 42 pages
Start with the answer and work backward.
I had some stickers. My friend gave me 14 more. Now I have 48 stickers. How many did I start with?
Work backward: − =
Started with: stickers
There were some birds on the tree. 12 more flew in. Now there are 35 birds. How many were there at first?
Work backward: − =
Started with: birds
Jake had some dollars. He earned $23 mowing lawns. Now he has $56. How much did he start with?
Work backward: − =
Started with: dollars
Challenge: Two-Step Backward!
The baker made some cupcakes. In the morning she sold 28. In the afternoon she sold 19. Now she has 33 left. How many did she start with?
Work backward from 33:
Step 1: 33 + 19 =
Step 2: + 28 =
Started with: cupcakes
Find the missing number by working backward.
+ 35 = 78
Work backward: − =
95 − = 42
Work backward: − =