here are nine mental math tricks for this curious about this type of things. tricks include:

• multiplying anything by 9, 99, or 9999
• multiplying any number by 11
• multiplying any number by 5, 25, or 125
• multiplying together two numbers that differ by a small even number
• squaring 2-digit numbers that end in 5
• multiplying together 2-digit numbers where the first digits are the same and the last digits sum to 10
• squaring other 2-digit number
• multiplying by doubling and halving
• multiplying by a power of 2