จาก 100 - 80 + 40 ?
http://www.eduplace.com/math/mathsteps/4/a/
Over time, mathematicians have developed a set of rules called the order of operations to determine which
operation to do first. The rules for order of operations are:
Do operations in parentheses.
Multiply and divide from left to right.
Add and subtract from left to right.
12 / 4 + 5 * 3 – 6
3 + 5 * 3 – 6 (since 12 divided by 4 = 3)
3 + 15 – 6 (since 5 times 3 = 15)
18 – 6 (since 3 + 15 = 18)
12 (since 18 – 6 = 12)