Total these measurements. your answer should indicate the proper accuracy. include units.
4.2 cm
6.11 cm
8.5 cm
i dont get how to do this

You are dealing with the decimal point '.'.  That means that you should add everything on the right first, then everything on the left.  Realize that .2 is actually .20, so you cannot add the values in decimals directly.  In this problem, you have .20, .11, and .50

So add on the right: 20 + 11 + 50 = 70 + 11 = 81, the first part is .81 – note the decimal in front of the ‘81’.

Since this number is less than 100, there is no carry over to the ones place.  Now you can add the three numbers that are all left of the decimal point directly: 4 + 6 +8 = 4 + 14 = 18.

Then you combine your two answers into 18.81.