First, we need to find the total number of parts in the ratio 1:4:5:
1 + 4 + 5 = 10
This means that the string is divided into 10 equal parts. To find the length of each part, we divide the total length of the string by the number of parts:
80 cm ÷ 10 = 8 cm
Now, we can find the length of the longest piece, which is 5 times the size of each part:
8 cm x 5 = 40 cm
Therefore, the length of the longest piece is 40 cm.