Need to Know
Money
The rules you must have ready for the exam.
How to use this: say each rule in your head, then tap Show me to check.
Percentages
1 ruleRule 01
Of / increase / decrease
6% of 250 = 0.06 × 250
Increase: ×(1 + p/100). Decrease: ×(1 − p/100).
Profit & error
1 ruleRule 02
Two formulas
% profit = increase/original × 100
% error = error/real × 100
Reverse percentages
1 ruleRule 03
Work back to 100%
If a price includes a 10% profit, it is 110%. Divide to 1%, then × 100.
Compound interest
1 ruleRule 04
The formula
F = P(1 + i)t
Find P: P = F ÷ (1+i)t.
Loans
1 ruleRule 05
With repayments
Add the interest each year; subtract any repayment before charging the next year’s interest.
Mathslive.ie · Money · Need to Know