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0 / 60 seg.
Mr. Smith has two children. If the older child is a boy, what is the probability that the other child is also a boy?
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Clue
TERRY IS 5 YEARS OLD. LET'S TRANSLATE WORDS TO MATH. "ALICE WAS FIVE YEARS OLDER THAN TERRY IS NOW" TRANSLATES TO: A = 5 + T, WHERE A IS THE AGE OF ALICE AND T IS THE AGE OF TERRY. NOW TRANSLATE AGAIN. "TERRY IS HALF AS OLD AS ALICE WAS" BECOMES : T = (1⁄2)A. SOLVING THE TWO EQUATIONS WILL GIVE T = 5
THE PROBABILITY IS 0.5. THE POSSIBILITIES ARE (GIRL, GIRL) AND (GIRL, BOY). SO THE PROBABILITY THAT BOTH CHILDREN ARE GIRLS IS 1/2 OR 0.5
THE PROBABILITY IS 1/2. THERE ARE INITIALLY 4 POSSIBILITIES: GIRL-GIRL, GIRL-BOY, BOY-GIRL AND BOY-BOY. SINCE THE OLDER CHILD IS A BOY, WE CAN RULE OUT THE GIRL-GIRL AND GIRL-BOY COMBINATIONS
THEY ARE 10.5 AND 0.5 YEARS OLD. A COMMON WRONG ANSWER IS 10 AND 1. THIS IS WRONG BECAUSE THE DIFFERENCE IN AGE WOULD BE 9, NOT 10
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