To divide a number in N into three parts in the ratio of a:b:c

first part = ×N
second part = ×N
third part = ×N

To divide a perimeter 60 into three sides in the ratio of 3:4:5

first side = ×60 = ×60 = ×60 = 15
second side = ×60 = ×60 = ×60 = 20 
third side = ×60 = ×60 = 25

Edwin