推 DreamKite:是3的倍數也是5的倍數,就是15的倍數 09/25 08:01
When positive integer n is divided by 3, the remainder is 2;
and when positive integer t is divided by 5, the remainder is 3.
What is the remainder when the product nt is divided by 15 ?
(1) n - 2 is divisible by 5.
(2) t is divisible by 3.
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【答案】C
【思路】
n=3x+2
t=5y+3
nt =15xy+10y+9x+6
(1) n-2 = 5a => 3x+2-2 = 5a => 3x is divisible by 5 => n=15b+2
(2) t = 5c => 5y is divisible by 3 => t=15d+3
(1)+(2)
nt = 15Z + 6
黃色部份不懂怎麼變換過去的
麻煩了......
thanks
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