We presume you want a fraction that is a pure number, so both units of time need to be the same.
... (16 days)/(4 weeks) = (16 days)/(28 days) = 16/28 = 4/7
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Comment on reducing fractions
Some calculators will reduce fractions for you. When you do it without benefit of a calculator, it helps immensely if you know your times tables. Here,
... 16 = 4×4
... 28 = 4×7
so ...
... 16/28 = (4×4)/(4×7)
... = (4/4)×(4/7)
... = 1×(4/7) = 4/7