Rounding decimals on a number line

MathematicsNumbers & ArithmeticAges 11–12

A decimal number sits on two number lines: the top one is marked off by the rounding place, and the bottom one zooms into the interval containing it, split into 10 parts. Drag the point on either line to change the number and see which mark it sits closer to; the deciding digit is highlighted, and the rounded result and the error |number − result| appear instantly.