Answer:
see explanation
Step-by-step explanation:
Given M is directly proportional to r³ then the equation relating them is
M = kr³ ← k is the constant of proportion
To find k use the condition r = 4 when M = 160, thus
160 = k × 4³ = 64k ( divide both sides by 64 )
2.5 = k
M = 2.5r³ ← equation of proportion
(a)
when r = 2, then
M = 2.5 × 2³ = 2.5 × 8 = 20
(b)
when M = 540 , then
540 = 2.5r³ ( divide both sides by 2.5 )
216 = r³ ( take the cube root of both sides )
= r , thus
r = 6