Number Power-Ups

Place Value to Billions

Millions, ten-millions, hundred-millions, billions.

Roman Numeral Arithmetic

Romans are easy if you visit Arabic-land in the middle.

Twin Primes & Co-primes

Twin = both prime, differ by 2. Co-prime = hcf is 1.

Divisibility Rules

Digit sum, last digit, last two digits, instant tests.

Cryptarithm Puzzles

Same letter = same digit. Crack it column by column.

Estimation of Products

Close-enough beats exact when speed matters.

Division Cryptarithms

Every line you subtract is the divisor times one digit.