Solved using overlapping sets:

Total No. of students taking algebra = a

Total No. of students taking geometry = g

Students taking both = 15

So from the information above,

Total No. of students taking only algebra = (a-15)

Total No. of students taking only geometry = (g-15)

Total No of students = (a-15) + (g-15) + 15 = 90;

So, a+g = 105

From the condition given, 2g = a, so substituting

a = 70 & Total No. of students taking only algebra = (a-15) = 55 %

Answer = C

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