63- 17-20 equals 26. Addition, subtraction, multiplication and division are called elementary arithmetic. Among them, addition and subtraction are called first-order operations, and multiplication and division are called second-order operations. Operation at the same level: (addition, subtraction, multiplication and division), calculated from left to right. Different levels of operations: (multiply, add, multiply, subtract, divide and add, divide and subtract) add first and then subtract, and add first and then subtract in brackets.