Multiplying Decimals

Ignore the decimal points and multiply the digits: $6\times 4=24$.

Now count the decimal places in the question: one in 0.6 and one in 0.4, so two altogether. The answer needs the same two decimal places, so 24 becomes 0.24.

A quick check: multiplying by a number less than 1 makes the result smaller, so the answer must be smaller than 0.4, and 0.24 is.

Ignore the decimal points and multiply the digits: $25\times 2=50$.

Now count the decimal places in the question: two in 0.25 and one in 0.2, so three altogether. Putting three decimal places into 50 gives 0.050, which is written as 0.05.

A quick check: 0.2 is one fifth, and one fifth of 0.25 is 0.05.

Ignore the decimal points and multiply the digits: $75\times 8=600$.

Now count the decimal places in the question: 2 in 0.75 and 2 in 0.08, so 4 altogether. The answer must have 4 decimal places, so 600 becomes 0.0600, which is written as 0.06.

Notice the answer is smaller than both numbers you started with: multiplying by a number less than 1 always makes a value smaller.

Ignore the decimal points and multiply the digits: $32\times 15=480$.

Count the decimal places in the question: 1 in 3.2 and 1 in 1.5, so 2 altogether. Give 480 two decimal places: 4.80, written as 4.8.

Sense-check: 3.2 multiplied by 1.5 should be about half as much again as 3.2, and $3.2+1.6=4.8$.

Ignore the decimal points and multiply the digits: $16\times 375=6000$.

Count the decimal places in the question: 1 in 1.6 and 2 in 3.75, so 3 altogether. Give 6000 three decimal places: 6.000, which is just 6.

Sense-check: 3.75 is close to 4, and $1.6\times 4=6.4$, so an answer of 6 is sensible.

Multiply the distance per litre by the number of litres: $15.2\times 8.5$.

Ignore the decimal points and multiply the digits: $152\times 85=12920$. Count the decimal places in the question: 1 in 15.2 and 1 in 8.5, so 2 altogether. Give 12920 two decimal places: 129.20, written as 129.2 kilometres.

Sense-check: $15\times 8.5=127.5$, so an answer close to 129 is sensible.