How old are Toby, Cindy, and Dennis?

Dennis is 10 years older than Toby, and Cindy is 5 years younger than Toby.  If the sum of their ages is 50, How old is Toby? How old is Cindy? How old is Dennis?

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x = Toby’s age
x + 10 = Dennis’ age   {Dennis is 10 years older than Toby}
x - 5 = Cindy’s age   {Cindy is 5 years younger than Toby}

Add the ages together, and set equal to 50.

x + x + 10 + x - 5 = 50   {the sum of their ages is 50}
3x + 5 = 50   {combined like terms}
3x = 45   {subtracted 5 from each side}
x = 15   {divided each side by 3}
x + 10 = 25   {substituted 15, in for x, into x + 10}
x - 5 = 10   {substituted 15, in for x, into x - 5}

Toby is 15
Dennis is 25
Cindy is 10

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