Mentally Multiply & Divide

You already do mental math!  123 x 10 =  1,230  123 x 100 =  12,300  123 x 1000 =  123,000 How is it different to do…  12.3 x 10 =  123  12.3 x 100 =  1,230  12.3 x 1000 =  12,300

All you do is move the decimal!

Working with ‘Big’ Numbers 10, 100, 1000, 10000, …  Multiplying :  Move the decimal point RIGHT the same number of times as there are zeros in the number!  Example: x 100 = 1,204.5  Dividing :  Move the decimal point LEFT the same number of times as there are zeros in the number!  Example: ÷ 100 =

Working with ‘Little’ Numbers 0.1, 0.01, 0.001, …  Multiplying :  Move the decimal point LEFT the same number of times as there are digits after the decimal point!  Example: x =  Dividing :  Move the decimal point RIGHT the same number of times as there are digits after the decimal point!  Example: ÷ 0.01 = 1,204.5

Remember: If you multiply by a BIG number, you move to the RIGHT. If you multiply by a LITTLE number, you move to the LEFT. When you divide, everything moves in the OPPOSITE direction.

Practice  7,845.2 x 1000 =  7,845,200

Practice  7,845.2 ÷ 1000 = 

Practice  7,845.2 ÷ =  7,845,200

Practice  7,845.2 x 0.1 = 

Practice  ÷ 10 = 

Practice  1900 x 1000 =  1,900,000

Practice  x = 

Practice  ÷ 0.01 =  0.4

Practice  x 1000 =  0.3

Practice  19 x = 

Practice  100 x =  1,352

Practice  9, ÷ 10,000 = 

Exit Ticket x 0.001= 2. 10,000 x 0.45 = ÷ 0.01 = ÷ 10 = x =