When I was 4 years old, my sister was half my age. Now am 100. How old is my sister? 

If your sister was half your age when you were 4 years old, she was 2 years old, meaning that she is 2 years younger than you and thus 98 when you are 100.

Expert Answers

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You can make use of equations.

For instance, let the number of years you need to grow to become 100 years old be x. Then

4 + x = 100

x = 100 - 4 = 96

When you are 4 years old, your sister is half your age, that...

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You can make use of equations.

For instance, let the number of years you need to grow to become 100 years old be x. Then

4 + x = 100

x = 100 - 4 = 96

When you are 4 years old, your sister is half your age, that is, 0.5 x 4 = 2 years old.

Your sister grows for 96 years to become 2 + 96 = 98 years, when you are 100 years old.

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Your sister is 2 years younger than you as when you are 4 she is 2(half  your age); therefore when you are 100 your sister is 98 years old.

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When you were 4 years old, your sister was half your age. Half of 4 is 2, your sister was 2 years old when you were 4 years old or she is 2 years younger to you.

Your present age is 100, as your sister is 2 years younger to you, she is 98 years old.

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