science

18 days is correct, but you must show your working.

Why? most managers are only interested in answers, not methods.

BTW 1 house per 30 man days--3 houses per 90 man days---3 houses per 18 days for Mexicans.

Anglo's will be a bit slower but the cleanup is faster. Anglos tend not to spray paint.

16 if its a polish crew, 20 if its a pommy crew (if they actually finish it).

Only 1 day if they are all brick veneer .

Eff it, let's try some ratios just for kicks...

We want to know how "time required" changes when "man-power" and "number of houses" change.

We are decreasing man-power from 12 to 5, so time required increases by a factor of (12/5).

We are decreasing the number of houses from 4 to 3, so time required goes down--it gets multiplied by (3/4).

(10 days) * (12/5) * (3/4) = (10 days) * (9/5) = 18 days.

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Just like the unitary method, ratios are another good approach problem-solvers should have hard-wired into their heads.
(Ratios can be "intuitive" as well.)

Yes, that is almost complete as a teaching package. You glossed over the "decision procedure" involved in whether the multiplying fraction (ratio) has the larger quantity on the top or bottom. That is what students sometimes do not find "intuitive". This is taught by asking the question "does this change increase or decrease the time" ....and to remind the student that for an increase, the larger number "goes on the top".

(Just to get back to the unitary method. I should have added that this inappropriate whenever more than a single pair of parameters is involved as is often the case for electricity costing, in which time is not usually held constant. And more generally, V, I, and R are often to be manipulated to determine the wattage, if the question were to be classified as "Science" rather than "Arithmetic".)