(a)

Given length of wire

5 small stars = 4 large stars

x 5

25 small stars = 20 large stars

(b) 

Given

20 large stars + 14 small stars = 12.48m

25 small stars + 14 small stars = 12.48m

small star needs 0.32m wire

big star needs 0.4m wire

Wire for remaining stars

(15 big stars and 6 small stars) = 0.4 x 15 + 0.32 x 6 = 7.92m