AP CSA FRQ Guide
2020 FRQ 2: CheckDigit
Validate a number with a rightmost check digit by separating the base number and check digit, then describe how to track valid-call counts.
Methods and Control Structures25 minute targetstatic methodsinteger divisionmoduloclass design
Skills Tested
What this FRQ is really practicing
static methodsinteger divisionmoduloclass design
Treat these skills as the study checklist. If any tag feels shaky, review that topic before attempting the full written response.
Starter Approach
How to begin without copying a solution
- 1Part A: remove the rightmost digit to get the base number and compare it with getCheck.
- 2Part B: explain how to add class-level state that tracks valid checks.
Write a first attempt before revealing any solution outline. Most AP CSA FRQ progress comes from tracing your own code and finding the missing case.
Common Mistakes
Mistakes to watch for while writing
Passing the full number including the check digit into getCheck.
Comparing against the leftmost digit instead of the rightmost digit.
Using a local variable for the count in the written design response.
Self Check
Review questions before you submit
Uses modulo to isolate the rightmost digit.
The check digit is the ones digit.
Uses integer division to remove the rightmost digit.
The remaining number is passed to getCheck.
Calls getCheck on the base number.
Do not reimplement the check calculation.
Written response mentions static counter state.
Tracking across calls needs class-level state, not a local variable.
Practice Links