=TEXT(SUMIF(B:B, "=x", A:A), "[h]:mm:ss")
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=TEXT(SUM(FILTER(A:A, B:B="x")), "[h]:mm:ss")
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=ARRAYFORMULA(TEXT(SUM(IF(B:B="x", A:A, )), "[h]:mm:ss"))
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=TEXT(SUM(QUERY(A:B, "select A where B ='x'")), "[h]:mm:ss")
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=TEXT(SUMPRODUCT((B:B="x")*(A:A)), "[h]:mm:ss")
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