Two persons A and B have a total sum of $ 3 6 0 . First, A gives 2 0 % of what he has to B . Then B gives 2 0 % of what he now has back to A . If the final amount of each person is the same as the starting amount, what was the initial amount that person A had?
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Let the initial and end amounts A and B have be a 0 and b 0 , and a 1 and b 1 . Then a 0 + b 0 = 3 6 0 . Then
{ b 1 = 0 . 8 ( 0 . 2 a 0 + b 0 ) = 0 . 1 6 a 0 + 0 . 8 b 0 a 1 = 0 . 8 a 0 + 0 . 2 ( 0 . 2 a 0 + b 0 ) = 0 . 8 4 a + 0 . 2 b 0
Since { b 1 = b 0 a 1 = a 0 ⟹ 0 . 1 6 a 0 + 0 . 8 b 0 = b 0 ⟹ 0 . 8 4 a + 0 . 2 b 0 = a 0 ⟹ b 0 = 0 . 8 a 0 ⟹ b 0 = 0 . 8 a 0
Therefore, a 0 + b 0 = 1 . 8 a 0 = 3 6 0 ⟹ a = 2 0 0
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Let initially A had $ x and B had $ ( 3 6 0 − x )
After A gave the money to B, he had $ 0 . 8 x and B had $ ( 3 6 0 − 0 . 8 x )
After B gave money to A, the later (A) has $ ( 0 . 6 4 x + 7 2 )
So, according to the given condition,
x = 0 . 6 4 x + 7 2 ⟹ 0 . 3 6 x = 7 2 ⟹ x = 0 . 3 6 7 2 = 2 0 0 .