Make a matrix..
..............M ......F........total
<30......t/2_.....15...t

So t/2+15=t......t/2=15....t=30%


Also if 50% are not females, the other 50% will be females and this is 15%of Total
So total will be 15*2=30%

STEP by STEP ( say there are 100 voters)
1) 15% of the voter turnout were under the age of 30 and were females.... so 15 voters are BOTH <30 and F.
2)50% of the voter turnout under the age of 30 were not females,..say Total voters <30 be x, so x/2 are males and x/2 are females
3) but we know that BOTH <30 and F =15=x/2...x=30
so 30% of total

see attached diagram
ssp4all please refer your below post

B

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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


GMAT online Tutor

Let The total number of people who voted be = T.

The number of people under of age of 30 who voted be = X.

Now given that 50% of people under the age of 30 were not females, which means 50% of the people under the age of 30 who voted were females.
Thus number of females under age 30 = 50/100 * X
= 0.5 X

Now we also know that the relation between total number and the number of females under 30 years who voted is = 15% of T
= 0.15 T

These two numbers are equal
0.15T = 0.5 X

X/T = 0.15/0.5

X/T = 3/10

X/T = 30/100

therefore % of people who voted and were under 30 years of age = 30% of T
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Taking GMAT on 16th August 2018. Aiming for ISB YLP and MiM programs in Europe. Please give kudos if you like something I do.