We can multiply two of the numbers together: let’s choose 3.6×8.5.
Write the numbers as (3+0.6)(8+0.5). We need to multiply like this:
3(8+0.5)+0.6(8+0.5) to make the arithmetic easier: 24+1.5+4.8+0.3. Note where the decimal point is.
When we add together just the decimal fractions we get 1.6, and when we add the whole numbers we get 29 so 29+1.6=30.6.
Now we take the remaining number and multiply by the result above: 0.54×30.6.
We write this as 0.54×(30+0.6)=0.54×30+0.54×0.6. 30 is 3×10 and to multiply by 10 we just move the decimal point of 0.54 one place to the right: 5.4 then we can multiply by 3: 0.54×30=5.4×3=16.2.
To multiply 0.54 by 0.6 we multiply 54 by 6=324. We add the number of decimal places from the two numbers: 0.54 has 2 decimal places and 0.6 has 1 decimal place, 2+1=3, so we take our product 324 and count three decimal places from the right to give us 0.324.
So we finally add 16.2 to 0.324 to get 16.524.