John asked in Science & MathematicsMathematics · 2 months ago

# how do I solve 24 X 0.85 using long multiplication without a calculator? ?

I know the answer is 20.40 but I keep getting 14.40. All the online resources are showing 92 when it comes to multiplying 8x4 but I know it's 32. What am I doing wrong?

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• Vaman
Lv 7
2 months ago

24*0.85=24*85/100=6*85/25=6*17/5=102/5

Multiply it with 20. 2040/100=20.4

• Amy
Lv 7
2 months ago

I have no idea what resource would tell you 8x4 is 92, nor how that could give you a smaller answer.

If you can write it out on paper, first multiply 24 by 85 and then put the decimal point back in.

Going digit by digit and placing the results in the correct positions:

20x80 = 1600

4 x 80 = 320

20 x 5 = 100

4 x 5 = 20

Therefore 24 x 85 = 2040

If you have to do the problem in your head, it's time for some simplifying tricks. I like to multiply a full-length (in this case two digits) number by the smallest digit in the problem, then build up from there.

2 x 0.85 is easy enough to do in your head even with the decimal point. It's 1.7.

Therefore 20 x 0.85 is 17.

4 x 0.85 is 2 x 2 x 0.85, or 2 x 1.7. That's 3.4.

Finally, 17 + 3.4 = 20.4

• 2 months ago

Ignore decimals at this point and multiply normally (remember how many decimal places you are ignoring. 0.85 has 2)

24 x 85 = 2040

Now add the decimals back in (put the 2 back)

20.40

• 2 months ago

There are 'two decimal places in the multiplicand', so there will be two decimal places in the answer.

Then complete long multiplication of 24 x 85

.85

x 24

____

1700   ( 85 x 20)

+ 340  ( 85 x 4)

____

2040

Place in two decimal places :   20.40  The Answer.

• Jim
Lv 7
2 months ago

Here's how I would do it:

• Sky
Lv 7
2 months ago

You can do it in your head.  What is one tenth of 24?  That's 2.4.  What's half of that?  1.2.  .85 is one and a half tenths less than 1, so subtract one and a half tenths of 24 from 24:  24 - 2.4 - 1.2 = 24 - 3.6 = 20.4

• 2 months ago

I promise that they're not showing you 92.  8 * 24 = 192

8 * (20 + 4) =>

8 * 20 + 8 * 4 =>

160 + 32 =>

192

• 2 months ago

24 × 0.85 = 24 - 3.6 = 20.4

• 2 months ago

First, ignore the decimal point and just do the long multiplication of two integers that you learned before.

. 24

x 85

------

4 * 5 is 20, so put the 0 and carry the 2:

. .₂

. 24x 85------. . .0

Then 2 * 5 is 10 plus the 2 is 12:

. .₂ . 24x 85------.120Next, we do the 4 * 8 which is 32, so put the 2 in the tens place and carry the 3:

. .₃ . 24x 85------.120. .2

2 * 8 is 16, plus 3 is 19:

. .₃ . 24x 85------ .120192----------  Add the results together:

2040

Now look back at your original problem and count the decimal places.  There are 2 total, so place the decimal point so there are 2 decimal places in your answer:

20.40

which would then simplify to:

20.4

Hope that helps.

• 2 months ago

Count the decimal places in the two numbers.  There are 2 in the second number and none in the first.  0 + 2 = 2 in all.

Then remove decimal places and multiply as a whole number problem:

2 4

x  8 5

----------

1 2 0

1 9 2

----------

2 0 4 0

Finally, put the decimal point back in.  The number of places is the total from the first step: 2 decimal places:

24 x 0.85 = 20.40

Obviously, you need to be up to speed on multiplying whole numbers for this to work.  There are other ways to do essentially the same thing, but I learned this when the calculator *was* a pencil (Ticonderoga #2, usually with a few teeth marks in it.  :^)  It always seemed easier to me to keep the decimals out of the calculation until the very end.