Task: Question Answering
Input (English): Passage: With the death of Admiral Stepan Makarov during the siege of Port Arthur in April 1904, Admiral Wilgelm Vitgeft was appointed commander of the battle fleet and was ordered to make a sortie from Port Arthur and deploy his force to Vladivostok. Flying his flag in the French-built pre-dreadnought Tsesarevich, Vitgeft proceeded to lead his six battleships, four cruisers, and 14 torpedo boat destroyers into the Yellow Sea in the early morning of 10 August 1904. Waiting for him was Admiral Tōgō and his fleet of four battleships, 10 cruisers, and 18 torpedo boat destroyers. At approximately 12:15, the battleship fleets obtained visual contact with each other, and at 13:00 with Tōgō crossing Vitgeft's T, they commenced main battery fire at a range of about eight miles, the longest ever conducted up to that time. For about thirty minutes the battleships pounded one another until they had closed to less than four miles and began to bring their secondary batteries into play. At 18:30, a hit from one of Tōgō's battleships struck Vitgeft's flagship's bridge, killing him instantly. With the Tsesarevich's helm jammed and their admiral killed in action, she turned from her battle line, causing confusion among her fleet. However, Tōgō was determined to sink the Russian flagship and continued pounding her, and it was saved only by the gallant charge of the American-built Russian battleship Retvizan, whose captain successfully drew away Tōgō's heavy fire from the Russian flagship. Knowing of the impending battle with the battleship reinforcements arriving from Russia , Tōgō chose not to risk his battleships by pursuing his enemy as they turned about and headed back into Port Arthur, thus ending naval history's longest-range gunnery duel up to that time and the first modern clash of steel battleship fleets on the high seas. 
Question: How many minutes after the time the battleship fleets obtained visual contact with each other did a torpedo strike Vitgefts flagship's bridge, killing him? 
Answer type: Number 
Output (English): 315.