Sums involving two-digit numbers or multiplication or subtraction were beyond him.

当加法的和是一个两位数或做乘法和减法时，查尔斯便无能为力。

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Based on sign digit negabinary representation, the two step addition and one step subtraction of arbitrary length operands can be performed.

基于符号数字负二进制表达，提出了任意字长操作数的并行两步加法与一步减法。

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This is helpful when parents know the specific subject the child is working on, like double-digit subtraction.

WSJ: The Digital Solution: A Review of LeapsterGS