Question

Lisa's and her dad's ages combined are 51. Her dad's age is 4 less than 4 times her age. How old is Lisa?

Answer 1

`L=9.4` years

or `9` years `3` months

Explanation:

Lisa's age: `L`
Dad's age: `D`
Lisa's age and Dad's age combined is 51: `L+D=51`
Dad's age is 4 less than 4 times her (Lisa's) age: `D=4L-4`

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You have a value for `D` that is `4L-4`, so substitute that into the other equation:
`L+\stackrel{(4L-4)}{\cancel{D}}=51\rightarrowL+(4L-4)=51`
Adding like terms, `5L-4=51`
Add 4 to each side, canceling out the left: `5L=51-4=47`
Dividing both sides by 5, canceling out the left: `L=47/5=9.4`

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9.4 years, or `9 1/4\leftrightarrow` `9` years `3` months