Unit 7 · Finding Procedures
Work Backwards — What Are the Digits?
Lesson 4 of 5
Premium
About this lesson
Hayden follows 3 instructions — choose a number, add 4, multiply by 2 — and ends up with 40. What is the digit sum of his starting number? The trick: you never need to guess! Work backwards step by step, undoing each operation in reverse.
Chapters
0:00Introduction
0:23The Question
0:59Mapping the Forward Operational Flow
1:23Executing the Inverse Operations
1:56Answering the Final Question
2:19Exam Strategy — The Premature Finish Trap
3:09Confirming the Final Option
Question
Hayden follows these instructions:
- Choose a starting number.
- Add 4 to your number.
- Multiply by 2.
He gets the answer 40.
What do you get if you add together the digits in his starting number?
Free account · OC
Take your OC prep further
- Sample questions & worked solutions across core OC topics
- Digital exam simulator with interactive practice sets
- Personalised progress tracking across all your devices
Already have an account? Log in here
No payment required. Compare OC plans
Give Your Child the Best Chance at OC Entry
Join NSW families preparing their children for the Opportunity Class Placement Test with the most realistic online OC practice tests available. First tests free—no credit card required.
Claim Your Free OC Practice Tests