Larry is 10 years old and his sister is 7 years old. To determine the number of years that it will take that the sum of their ages will be 25, assign a variable to represent the time in years. Let it be x.

So after x years, their ages...

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Larry is 10 years old and his sister is 7 years old. To determine the number of years that it will take that the sum of their ages will be 25, assign a variable to represent the time in years. Let it be x.

So after x years, their ages will be:

Larry's age = 10 + x

His sister's age = 7 + x

Then, add their ages and set it equal to 25.

`10 + x + 7 + x = 25`

And, solve for x.

`17 + 2x = 25`

`2x=8`

`x=4`

**Therefore, the sum of Larry's age and his sister's age will be 25 in 4 years.**