译文:
一个两位数是它的各个数位上数字之和的4倍.如果把它的个位数字和十位数字交换,它会增加到21,求这个两位数.
解法:
设这个两位数的个位和十位分别为y,x,即这个数是10x+y
10x+y=4(x+y)
21=10y+x
解得:x=1,y=2
这个两位数是12
A two-digit number is 4 times the sum of its digits.If the d
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