Anna is 4 years older than her brother jim. the sum of their ages is 25 years. what is jim's age?

Let's say Jim's age is x
Anna's will be x +4 cuz she is 4 years older than his age
the sum of their ages is 25
to make an equation
(x+4) + x = 25
combine like terms
2x + 4 = 25
subtract 4 from both sides 
2x = 21
now isolate the variable x (Jim's age)
x = 10.5
Jim is 10.5 

(Anna is [x+4 so (10.5 + 4) that is] 14.5)

Hope this helps ;)

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