Ateacher is having three students take care of 28 goldfish during summer. he gave some of them to alaina. then he gave twice as many to miguel. he gave twice as many to kira as he gave to miguel. how many fish did each student get. .

To solve this problem you create an equation that equals 28 and had all students in terms of Alaina.  Alaina can be represented with x.
Miguel has twice as many as Alaina so he has 2x fish.
Kira has twice as many as Miguel so 2*2x = 4x

Alaina (X) + Miguel (2X) + Kira (4X) = 28 fish

combine like terms 
7x=28  

Divide both sides by 4
x=4

Alaina has 4 fish (x = 4)
Miguel has 8 fish (2x = 2*4 = 8)
Kira has 16 fish (4x = 4*4 = 16)