“Any student executes tens of elementary calculations daily. Over a lifetime, we must solve more than ten thousand multiplication problems. And yet, our arithmetic memory is at best mediocre. It takes a well-trained young adult considerable time, often more than 1 second, to solve a multiplication such as 3 × 7. Error rates average 10 to 15 percent. On the most difficult problems, such as 8 × 7 or 7 × 9, failure occurs at least once in every four attempts, often following more than 2 seconds of intense reflection.” MathsFun Fact Book:The Number Sense: How the Mind Creates Mathematics Source: The Number Sense: How the Mind Creates Mathematics