To solve this problem you must apply the proccedure shown below:
 1. You have that the largest doll is 12 inches tall. The height of each of the other dolls is 7/10 the height of the next larger doll. Therefore, you have:
 We don't know the number of dolls, so:
 x=12(7/10)^(n-1)
 Where x is the height of the doll and n is the number of the doll
 The answer is: x=12(7/10)^(n-1)