Note: In Maple 2015 and later versions, units are not surrounded by double brackets.
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$\mathrm{with}\left(\mathrm{Units}\[\mathrm{Natural}\]\right)\:$

In the first example, both $\mathrm{ft}$ and $s$ are interpreted as units, and they are subject to basic arithmetic. This causes the result to be converted to the default unit system.
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$\frac{3\mathrm{ft}}{s}$

$\frac{{1143}}{{1250}}{}\u27e6\frac{{m}}{{s}}\u27e7$
 (1) 
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$\mathrm{Units}\[\mathrm{UseSystem}\]\left(\mathrm{strict}\right)\:$

After setting strict use of the default system, $\mathrm{ft}$ is not interpreted as a unit anymore (it is not part of the default SI system).
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$\frac{3\mathrm{ft}}{s}$

${3}{}{\mathrm{ft}}{}\u27e6\frac{{1}}{{s}}\u27e7$
 (2) 
If you want to use the unit $\frac{\mathrm{ft}}{s}$ for one quantity, but generally work with the SI system, you can use the Unit command to achieve this.
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$3\mathrm{Unit}\left(\frac{\mathrm{ft}}{s}\right)$

${3}{}\u27e6\frac{{\mathrm{ft}}}{{s}}\u27e7$
 (3) 