Foundation
USE WHEN
A group composition changes after additions/removals and final percentage is given.
BEST METHOD
Equation-in-Total Method
TARGET
90–120 sec
MASTERY
not started
A group composition changes after additions/removals and final percentage is given.
Let original total or subgroup count be a variable and model the changed denominator carefully.
If only one group changes, keep the unchanged group count fixed.
Let original total be N and express subgroup counts in terms of N.
Timed drill
Identify the base or invariant before you calculate.
Attempt first · Standard Example
Attempt first · CAT-Level Example
3-level practice ladder
Foundation
CAT Standard
Advanced / Hybrid
cat 2025 slot 1: percentages of girls and boys leave a class; remaining counts impose an integer constraint.