American 16-year-old Cooper Lutkenhaus vowed to come back stronger after being eliminated in the 800 metre heats at the World Athletics Championships on Tuesday, comparing it to "failing a math test".

The Texas high-school student became the youngest runner to represent the United States at a world championships but made an early exit after finishing seventh in his heat in Tokyo.

Lutkenhaus looked in position to attack with about 150m to go but was unable to break through and crossed the line in 1min 47.68sec.

He took the positives from his performance in front of over 50,000 fans, saying he "came out here just to learn as much as I could and race the best guys in the world".

"It's not your day every time, anybody can tell you that," said Lutkenhaus, who does not turn 17 until December

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