The ratio of the volumes of the two spheres is 27: 343 and the sum of their radii is 10 units. find the radius of the smaller sphere.

R + R = 10
4/3 r³ π : 4/3 R³ π = 27 : 343
r ³ : R ³ = 27 : 343  / ∛
r : R = 3 : 7
3 R = 7 r
R = 7 r / 3
r + 7 r / 3 = 10   / · 3
3 r + 7 r = 30
10 r = 30
r = 30 : 10
r =  3
the radius of the smaller sphere is 3.