Let us assume the potential of each small drop = Kq/r = V
The volume of n (here 27 small drops) drops = Volume of one big drop.
n * 4/3 pi r³ = 4/3 pi R³
here r is the radius of a small drop
R is the radius of the big drop.
Therefore , R = n⅓ * r
here charge remains conserved.
charge on 1 small drop = q and charge on big drop containing n drops = nq .
The potential on big drop = K (nq)/ (n⅓)r
= (n⅔) Kq/r
= (n⅔) V
Now substitute the n with the number of drops you wish to combine and do the math.
Potential of each small drop = 10V.
No. of drops: 27.
Potential on big drop = 10*K(27q)/(27⅓)r.
=(10 * 27)/3 Kq/r
=(10 * 27)/3 V
= 90 V