In how many years will I be twice as old ...?

it's easy.
If you are 55 and your daughter is 21
then you gave her birth 34 years ago.
Then when she will be 34 you will be just her double, isn't it??

So she will take another 13 years to get 34 and then you will be then 68, just double of 34.

so, it's 13 years, after that you will be just double of her age.

How old were you when you gave birth? now double it.
Otherwise I would need to know your birthdays.

55-21=34?
34 x 2=68

So you'll be 68, and she'll be 34, the same age as you when you gave birth. After that, you to will always be closer in age relatively. So when she's 68, she'll be 2/3 your age. Or you'll only be 1/2 as older. Confusing, huh?

If it makes you feel better, when I am half my Dad's age, I'll be 55, and he'll be 110!

after 13 years

let x= the time it will take

since the age is 55 we use 55+x

so if we add x to the mother's age we also add x to the daugter's age making it 21+x

thus we have the equation 55+x=2(21+55)

55+x=2(21+x)
55+x=42+2x
55-42=x
13=x

checking
21+13=34
55+13=68

34*2=68

For all such age problems the solution is linear and easy!

Say it will take 'x' years for your daughetr to be half of your age from now!

i.e (55 + x) = 2 times (21 + x)
or 55 + x = 42 + 2 x
or x = 13 years

So in 13 years she is be half of your age.
i.e she will be 21+13=34 and you will be 55+13=68!

Simple!

Let's assume that you will be twice as old as your daughter after " x " years.

After x years -
Your age will be (55 + x) and your daughter's age will be
(21 + x ) . But according to problem -

55 + x = 2 ( 21 + x)
ie x = 13 years

Hence after 13 years you will be twice as old as your daughter?

55 + x = 2(21 + x)
=> x = 55 -- 42 = 13
after 11 years you will be 55 + 13 = 68 and you daughter will be 21 + 13 = 34.
after 13 years you will be twice as old as your daughter.

55+x=2*(21+x)
x=13 years

13

let x be the number of years passes.

so in x years, you will be twice as old as your daughter
you = 2*daughter

your age in x years is 55 + x
your daughter's age in x years is 21 + x

55 + x = 2(21 + x)
55 + x = 42 + 2x
x = 13 years <== answer

check:
55 + 13 = 68

21 + 13 = 34

68 is twice as much as 34

So you want to know when you (55) will be twice as old as your daughter (21). Solve the equation.
55 + x = 21(2)
55 + x = 42
x = 42 - 55
x = -13

Sorry, it won't happen for you.