how do you find the answer to 4.8x0.23

Question

We know how to multiply all of our other problems. But when we do this one the same way, the answer comes out different. What are we doing wrong?

Answer 1

I think Julia has given a great answer.

I would add that when doing calculations (whether by hand or with a calculator), if you have time, it is often a good idea to do a quick mental check whether your answer seems reasonable by comparing it to what you would expect if you just guessed using rough calculation.

For example, this question could be read like this : "What is the product of some number a little less than five and another number almost as big as one quarter?"  When viewed this way, the question is almost the same thing as "What is five divided by four?" ... because multiplying by one over a value is equivalent to dividing by that value.

Keeping in mind that because both of the actual factors were smaller than this rough equivalent we'd expect our answer to be a little smaller than 1.25.  If you had put the decimal point in the wrong spot, you won't be close to that (you'll typically be off by at least a factor of roughly ten) and at least with this type of common mistake, you'd be alerted that something was wrong.

The best thing is to avoid mistakes in the first place, but training your ability to detect mistakes intuitively can also pay off.

Answer 2

The trick to multiplying decimals is to line them up and multiple like a normal problem. The only difference is at the end you count how many numbers are behind the decimal in both numbers then count from the end of the answer you got to place the decimal in the correct place.  

   4.8
X 0.23
    144   multiply 3*8 carry, then 3*4 add in carried number
+  960   0 as place holder multiply 2*8 carry, multply 2*4 add in carried number
  1104   add together 144 and 960

now here is the last step. There is an 8 (in 4.8 that counts as one decimal place) a 2 and a 3 (in 0.23 that counts as two decimal places) for a total of  3 numbers behind the decimal between the two numbers. so count from right to left in 1104 (your answer from multiplying) three decimal places and that is where you will put the decimal. 1  1  0  4 so the answer is 1.104

                                    

Hope this helps!

 

Julia