Word explanation in main comments section.
Model representation and ratio :
● →|A|C|C|C|
▲→|A|B|B|B|
● : ▲: (their difference)
5 : 3 : 2
shaded of ● →|C|C|C|
shaded of ▲→|B|B|B|
(shaded of ●) : (shaded of ▲) : (their difference)
5 : 2 : 3
The difference is unchanged/constant. So make the units the same.
Common multiple of 2 and 3 is 6 (2 x 3 = 6, 3 x 2 = 6)

● : ▲: (their difference)
5 : 3 : 2
15 : 9 : 6
(shaded of ●) : (shaded of ▲) : (their difference)
5 : 2 : 3
10 : 4 : 6
So now we have all the units the same size.
Circle area = 15 units = 120cm²
1 unit = 120cm² ÷ 15 = 8cm²
Unshaded area = 15 units - 10 units
= 5 units = 5 × 8cm² = 40cm²
Or
Unshaded area = 9 units - 4 units
= 5 units = 5 × 8cm² = 40cm²
b)
Shaded area = 10 units + 4 units = 14 units
Ratio of shaded to unshaded = 14 : 5