540/9 = 60cm
408/10 = 40.8
60-40.8 = 19.2cm longer
87 + 34
87 rounded = 90
34 rounded = 35
90 + 35 = 125
or
87 rounded = 90
34 rounded = 30
90 + 30 = 120
when rounding you are only getting an estimate so it is not going to be exact however you do it.
