It can be done 'in your head'! Multiply by 16 by multiplying by 8 and doubling.
Multiply by 24 by multiplying by 12 and doubling.
Multiplying by 15 can be done mentally in three stages. Multiply by 10. Find half the answer. Add the two parts.
Remember, each digit moves one place to the left when multiplying by 10.
Multiplying decimals by 100 is easy - just move each digit two places to the left, including moving across the decimal point.
Doubling decimals needs a good knowledge of doubling 2-digit numbers. Quite tricky!