Foundation
USE WHEN
Income, spending and savings percentages change and one original proportion is unknown.
BEST METHOD
Variable Equation Method
TARGET
60–90 sec
MASTERY
not started
Income, spending and savings percentages change and one original proportion is unknown.
Savings is the residual after expenditure: S = I − E.
Normalize original income to 100 or use variables I and E if percentages are asymmetric.
Let income = I and expenditure = E. Use S=I−E to build equations cleanly.
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 2024 slot 1: income ratios, different rent shares, later income increases, fixed total rent.