First of all place value and value are two different things. Place value is the position of a number and value is literally the value of a number. So the place value of 731.8 is tens because the order goes units, tens, hundreds etc, but the value is 30 because as said before it is in the tens place value. And as you asking the value of the 3 in 731.8, it is 3x10, which is 30.
