Financial Maths

Simplify by dividing both sides (same units first). Divide an amount: total parts, one part = total ÷ parts. Scale: model→real multiply, real→model divide.

Percentage of an amount: × the decimal. Increase: ×(100+x)%. Decrease: ×(100−x)%. Round money to 2 d.p.

One number as a % of another: first÷second×100. % profit/loss over the cost. % error over the real value.

F = P(1+i)ₜ for the future value; P = F÷(1+i)ₜ to work backwards. i is a decimal. APR = borrow, AER = invest; a changing rate is done year by year.

Gross tax = rate × income. Net tax = gross tax − credits. Net income = income − net tax. Two bands: up to the cut-off at the standard rate, the rest at the higher rate.

€1=$x: €→$ multiply, $→€ divide. Between two non-euro currencies, go through the euro. Find a rate from two prices of the same item.