1. The problem statement, all variables and given/known data Mass , r, of a round object grows in proportions with its SA. The density of the object is constant. Find the differential equation for the change in 'r' of the object over time. Arbitrary constants can be combined but final answer must depend on only r and constants. 2. Relevant equations Mass is proportional to volume. 3. The attempt at a solution I have, density (D) = r/V (r= mass) r = (D)*V This shows mass is proportional to volume. SA = 4πR^2 V = (4/3)πR^3 Isolate R in SA equation. Lets call SA = A A = 4πR^2 r = (A/4π)^0.5 Sub into V V = (4/3)π(A/4π)^1.5 dR/dt = r*V = r*(4/3)π(A/4π)^1.5 However I still have A in the differential equation. Am I on the right track?