Choose to add 3-digit numbers using formal written methods when appropriate

Students are encouraged to develop their arithmetic skills by choosing the most appropriate method, mental or formal written, when adding three-digit numbers. The ArithmeKit provides a series of calculations to practice both methods. For instance, adding 438 to multiples of 50 and 100, such as 438 + 200, can be easily computed mentally by focusing on the hundreds and tens place. Similarly, adding 324 to numbers like 300 and 270 can be done in one's head, by simply increasing the hundreds place and adjusting the tens and ones if necessary. However, when the numbers become more complex, such as 438 + 287 or 324 + 297, a formal written method might be more efficient to avoid errors and ensure accuracy.

Further exercises prompt students to reflect on the efficiency of different methods for various calculations. For example, simple additions like 123 + 456 or 222 + 444 might be tackled mentally by some, while more challenging sums such as 999 + 999 or 512 + 476 would benefit from the precision of a written method. The kit also includes problem-solving tasks, where students must determine the missing digits to make statements true, using each of the digits from 0 to 9 once. In another activity, students must convince a character named Colin that a written method is not only necessary when adding two three-digit numbers with a ones digit of 7, but also efficient for calculations like 367 + 476, by demonstrating the process and outcome of such an addition.