Have some Numeric formula fields and a Duration formula field

What I am trying to accomplish is converting the numeric formula fields to be duration fields. So that I can then multiply the converted numeric field with a duration field. 

[REF Units Remaining] is a numeric formula field
[Normal hrs/weeks] is a numeric formula field 
[# of weeks left on REF] is a duration formula field

I tried setting it up like this: Hours([REF Units Remaining] - (Days([Normal hrs/weeks]) * [# of weeks left on REF]))

The error I'm getting is on the asterisk, saying The operator "*" can't be applied on type of duration, duration

I need the two right fields multiplied together then subtracted by the left field.

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Alex Bartholomew
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