panjang tali semula adalah..

u15 = a+14
u3 = a+2b=18
u8= a+7b=43

(a+7b=43)
a+2b =18 (-)
5b =25
=25/5
b=5

a+2b=18
a+2(5)=18
a=10=18
18-10=8
a=8

s15= n/2 (2a+(n-1)b)
= 15/2(2.8+(15-1)5)
= 7,5(16+(14)5)
= 7,5 (16+70)
=7,5(86)
645 cm