Mental arithmetic up to 20: Addition and subtraction for 1st grade. 62 practice sets with mental math problems for addition and subtraction up to 20 in task types such as 17 + 3 = ? and 17 + ? = 20 or ? - 3 = 17, which children often find challenging at first. Mental arithmetic is an important part of elementary school mathematics. Children who do not practice mental arithmetic sufficiently usually struggle in other core areas of math, especially with written calculation methods. By engaging intensively with this workbook, mathematical difficulties can be effectively addressed in a didactic and methodological way. The workbook includes exercises for both strong and weaker math students. As they work through it, children will quickly notice an improvement in their mental arithmetic skills and will be pleased with the confidence they gain.