2m 40cm + 5m 25cm - 3m 80cm

Basically, the measures are like different variables because they have different values. So, first we want to add the meters.

2m+5m-3m=4m

Then, do the centimeters.

40cm+25cm-80cm=-15cm. 

You will get 3.85 m in total

Excellent answer Mellie :)

This answer is going in the wrap :))