Interest

Interest is what money earns for being lent out — or costs for being borrowed. Simple interest is the straightforward version: the same amount is earned every single year, because it is always worked out on the ORIGINAL sum.

Three things decide it. The principal P is what you started with. The rate r is the percentage earned per year. The time t is how many years it sits there. Multiply all three: I = P × r% × t.

Put £400 in at 5% for 3 years. Five percent of 400 is 20, so it earns £20 each year, and over three years that is £60. That is the interest — the account now holds £460, but the question asked what it EARNED, not what it holds. Read carefully which one is wanted.

Real bank accounts usually pay compound interest, where each year's earnings start earning too, so the total creeps ahead of this. Simple interest is the honest first model, and it is still exactly how many loans are quoted.

Find one year's interest, multiply by the number of years, and type the total interest earned.