6 hours
The two hoses together take 1/3 the time (4/12 = 1/3), so the two hoses together are equivalent to 3 of the first hose.
That is, the second hose is equivalent to 2 of the first hose. Two of the first hose could fill the vat in half the time one of them can, so 6 hours.
The second hose alone can fill the vat in 6 hours.
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The first hose's rate of doing work is ...
... (1 vat)/(12 hours) = (1/12) vat/hour
If h is the second hose's rate of doing work, then working together their rate is ...
... (1/12 vat/hour) + h = (1/4 vat/hour)
... h = (1/4 - 1/12) vat/hour = (3/12 -1/12) vat/hour = 2/12 vat/hour
... h = 1/6 vat/hour
so will take 6 hours to fill 1 vat.
