the question doesn't say how many dolls. there would be an infinite number.
But we'll just form the equation for the nth doll.
Largest doll = 12 inches
2nd doll = (12) x 0.7
3rd doll = (12 x 0.7) x 0.7
4th doll = [(12 x 0.7) x 0.7) x 0.7
OK, We now see the pattern. Each successive doll is mult by the previous
doll height x 0.7
the Nth doll, 12 is mult by 0.7(n-1)
So, the height of the nth doll can be expressed as: 12 x 0.7(n-1)
example: 4th doll = 12 x 0.7(4-1) = 12 x 0.73
10th doll = 12 x 0.7(10-1) = 12 x 0.79 etc
