Java gives you arithmetic operators for basic math: + (addition), - (subtraction), * (multiplication), and / (division). They work exactly like you'd expect from a calculator.
int sum = 10 + 3; // 13
int diff = 10 - 3; // 7
int product = 10 * 3; // 30
int quotient = 10 / 3; // 3 (not 3.33!)
Notice the last line. When both operands are int, Java performs integer division and drops the decimal portion. That 10 / 3 gives you 3, not 3.33. This catches beginners off guard constantly, and I'll explain why in the next unit.