Answer:
5.1 cm
. . . . . .
All we have to do is convert our 51mm to centimeters.
We divide the length value by 10 to get our centimeters.
51 divided by 10 = 5.1
All we have to do is move our "invisible decimal"
In my way, every whole number has an invisible decimal at it's end.
For ex: '60.'
We can get into exponents. 10 is 10¹, any exponent is the number of zeros at the end of a number may have. Ex: '10² = 100'
We can move our decimal to the left since we are dividing. The amount of exponent numbers are the amount you move it, multiplying and dividing are different things with that.
Let's get back on topic.
Moving our decimal to the left with dividing is 5.1 since 10¹ has a "1" as it's exponent, making us move it "1" to the left.
Multiplying is going the right way.
I hope this helps.
