Speed Addition

Mental Math · Axiom Academy

The fast way to add in your head is not to add faster — it is to reshape the problem into a couple of easy steps. Here is a first look. The way most of us were taught to add — stack the numbers, work right to left, carry a digit when a column spills over — is reliable, but it asks you to hold a running carry in your head. Mental-math techniques take a different route that skips the carrying. Watch 76 + 48 get added two ways at once. The school lane carries a digit twice; the fast lane rounds 48 up to 50 , adds, and gives the 2 back. Both reach 124 — but one of them gets there in fewer steps. Fewer steps, and no carry to keep track of — that is the whole reason the techniques feel fast. When the numbers are close, double the middle Here is a technique for a special case. Drag the two sliders to pick two numbers. Whatever bit one number sits above the halfway point, the other sits the same bit below it — those cancel — so the sum is simply twice the middle value. Try 47 + 49 : both are one away from 48 , so the total is . The closer the two numbers sit, the smaller the leftover bits — and the easier the doubling. When they are far apart, it still works, but a friendlier method usually wins. A lightning way to check your answer

This is the written version of the interactive lesson above. See the full Mental Math course.