Multiplication of Decimals

Starting from whole number times one decimal place in column form, the work adds decimal places stage by stage, through one place times one place and two places times two places. Alongside two-term and three-term calculations and Fill in the Blank, word problems on Density and Yield, Decimal Multiples (△ Times ○), Per-Unit Quantity and Area put the skill to use.

Placing the decimal point in the product to line up with the number being multiplied is a frequent error. If 2.4 × 1.3 comes out as 31.2 rather than 3.12, this is the pattern. Children also stumble over the fact that multiplying by a number smaller than 1 leaves a product smaller than what you started with.

Only one thing changes: the multiplier stops being a whole number and becomes a decimal. Counting digits past the point to fix where it lands in the product is a skill already gained in Multiplying and Dividing Decimals in grade 4. It carries into the unit that follows, Division of Decimals, where the same counting decides where the quotient's point goes.