As a result of media and parental attention to the need for improvement of core skills, this is a fast-paced mental arithmetic book. It is a sequel to the Human Calculator books 1 and 2, and is intended to help children to excel at adding, taking away, multiplying and dividing in their heads. In these books, children will have met new mathematical ideas, such as square roots, square numbers, fractions, multiplying by 10 and dividing by 100, and percentages. This book extends these number skills, enabling children to perform complex mathematical gymnastics in their heads. They will need scrap paper, a watch that can count in seconds and a calculator.