Joel has twice as many cds as mariella has. subtracting 7 from the number of cds joel has and dividing by 3 equals the number of cds blake has. if blake has 25 cds, how many cds does mariella have?

J = number of cd's joel has
m = number of cd's mariella has
b = number of cd's blake has

from problem statement, develop equations:
j = 2m
b = (j - 7)/3 = 25
m = ?

solve for m

b = 25 = (j - 7)/3
25 = (j - 7)/3
25*3 = j - 7
75 + 7 = j
j = 82

j = 2m =  82
m = 82/2
m = 41

thus
mariella has 41 cd's
joel has 82 cd's
blake has 25 cd's

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