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jmc

number theory senior

Problem

For how many pairs of consecutive integers in is no carrying required when the two integers are added?
Solution
Consider what carrying means: If carrying is needed to add two numbers with digits and , then or or . 6. Consider . has no carry if . This gives possible solutions. With , there obviously must be a carry. Consider . have no carry. This gives possible solutions. Considering , have no carry. Thus, the solution is .
Final answer
156