639.745 rounded to the nearest tenths place would be :
= 639.7
The tenths place in decimals is the first digit to the right of the decimal point. For example, 15.2 is fifteen and two tenths.
639."7"45..... 4 isn't high enough to change the 7 to an 8 so we keep it as it is.
So the correct answer is B.
Hope this helps,
Davinia. xo
