A standard algorithm is a specific method of computation that is conventionally taught for solving particular mathematical problems, such as addition, subtraction, multiplication, or division of multi-digit numbers. These methods vary somewhat by nation and time, but generally include exchanging, regrouping, long division, and long multiplication using a standard notation, and standard formulas for average, area, and volume. The standard algorithm is called "standard" because it is the general, widely-used step-by-step process used to add, subtract, multiply, or divide. Students learn these algorithms in elementary school and carry them with them throughout their school careers and into adulthood. While students alternative algorithms are often just as correct, efficient, and generalizable as the standard algorithms, the National Council of Teachers of Mathematics (NCTM) has made more explicit the need for learning basic math facts and correct, efficient methods.