Percents and Consumer Math

Percent basics

Percent equation

part = percent (as a decimal) \times whole

Percent proportion

\frac{part}{whole} = \frac{percent}{100}

Percent change

% change = \frac{new - original}{original} \times 100 %

Percent error

% error = \frac{| estimate - actual |}{actual} \times 100 %

Tax, tip or markup at rate r

final = original \times (1 + r)

Discount at rate d

sale price = original \times (1 - d)

Two discounts in a row

final = original \times (1 - d_{1}) (1 - d_{2})

Interest

I = P r t

P principal, r rate per year, t years

Total balance

A = P (1 + r t)

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