Adding in your head

Two tricks make adding in your head easy:

• Group equal numbers and multiply: 40 + 40 + 40 + 40 is just 4 × 40 = 160.
• Add place by place: 265 + 375 = (200 + 300) + (60 + 70) + (5 + 5) = 500 + 130 + 10 = 640.

In the book, the middle column holds 25,000, 400, 13,000, 1,500 and 60,000. Add them to make other numbers, using each as many times as you like:

• 38,800 = 25,000 + 400 × 2 + 13,000
• 3400 = 1500 + 1500 + 400

Can we make 1,000? The only middle number smaller than 1,000 is 400. Adding 400s gives 400, 800, 1,200, …, which jumps right over 1,000. So 1,000 cannot be made.

The book also lets us subtract. With 40,000, 800 and 300 we can make 39,800 = 40,000 − 800 + 300 + 300. Check: 40,000 − 800 = 39,200, then add 300 twice.

Grouping helps with number patterns too. One of the book's patterns has twelve 40s and ten 50s. Group them: 12 × 40 = 480 and 10 × 50 = 500. Total: 480 + 500 = 980.