The Perimeter of a Triangle

The missing measurment for the triangle's perimeter is 6.9 cm. Here is how I figured it out:


5cm
+41mm
-------
9.1 cm

I know that 10 mm make 1 cm, so, 41 mm is 4.1 cm. + 5 cm = 9.1cm. Which means, to get to 16 centimeters, you have to add 6.9 cm to find the perimeter of the triangle...

9.1 cm
+ 6.9 cm
-----
1 6 cm

Now, that's 16 cm. So, in mm, it's 160mm. basically, instead of adding 5 + 4.1, you'd add
50 mm + 41mm.
And, instead of adding 9.1 + 6.9, you'd add 91mm + 69mm.
So, that's basically the perimeter of the triangle. 16 cm and 160 mm.