Fix mixed arithmetic rounding

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.74359% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.44%. Comparing base (52cab70) to head (11f9329).

Files with missing lines	Patch %	Lines
src/number.rs	89.74%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   78.36%   78.44%   +0.07%     
==========================================
  Files          28       28              
  Lines        3638     3730      +92     
==========================================
+ Hits         2851     2926      +75     
- Misses        787      804      +17     

Flag	Coverage Δ
unit	78.44% <89.74%> (+0.07%)	⬆️

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