Answer:
M = 20
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 when r = 4, M = 160 , then
160 = k × 4³ = 64k ( divide both sides by 64 )
[tex]\frac{160}{64}[/tex] = k , then
k = 2.5
M = 2.5r³ ← equation of proportion
when r = 2 , then
M = 2.5 × 2³ = 2.5 × 8 = 20
