A boy is 4 years old and his sister is three times as old as he is. When the boy is 12, how old will his sister be?
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18
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16
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20
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21
To solve this question, the user needs to know basic arithmetic, the concept of multiplication and subtraction.
Let the sister's age be x.
Given, the boy is 4 years old and his sister is three times as old as he is. Therefore:
x = 3(4) = 12
So, the sister's current age is 12 years.
When the boy is 12 years old, it means that 8 years have passed since he was 4 years old. At that time, his sister's age would be:
x + 8 = 12 + 8 = 20
Therefore, the correct answer is:
The Answer is: C. 20.
The boy is 4 and his sister is three times his age, so she is 12 -- an age gap of 8 years. Age gaps stay constant over time, so when the boy turns 12 (8 years later), the sister will be 12 + 8 = 20. The other options ignore that the 8-year gap must be preserved, incorrectly trying to keep the '3x' ratio constant (which only held at that one point in time).