Math.

Fourth class. Budgeting is arithmetic: income minus expense, a savings percent, doubling an amount.

JS operators: + - * /. Order is school order: * and / before + and -. Use parentheses when you want a clear order: (income - expense) / 2.

Store the result in a variable:

const left = income - expense; console.log(left);

Mini example:

console.log(100 - 50);

Common mistake: concatenating strings with + instead of adding numbers; or forgetting that division can return a decimal (10 / 4 → 2.5).