Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: in a 100 m race , sam beats john by 5 seconds . on the contrary , if sam allowed john to start 35 m ahead of sam , then sam and john reach the finishing point at the same time . how long does sam take to run the 100 m race ?
13 seconds
10.5 seconds.