Volume of a sphere:
[tex]V = \frac{4}{3}\pi r^3[/tex]
Combining dough is like combining volume. So we get
[tex]V_3=\frac{4}{3}\pi r_3^3=\frac{4}{3}\pi r_1^3 + \frac{4}{3}\pi r_2^3 = \frac{4}{3}\pi (r_1^3+r_2^3)[/tex]
Where [tex]V_3 [/tex] is the combined volume of dough of radii [tex]r_1[/tex] and [tex]r_2[/tex]
The new radius is therefore
[tex]r_3^3=r_1^3+r_2^3[/tex]
Substituting values we get
[tex]r_3^3=3^3+5^3=27+125=152[/tex]
[tex]r_3=\sqrt[3] 152 \approx 5.34[/tex]
So the new dough is 5.34 cm, just a bit larger.
Cylinders:
[tex]V=\pi r^2 h[/tex]
So volume is directly proportional to height, if radius is constant. So ratios stay the same. 75/25=3 and the volume is also 3 times larger (ratio 3:1).
