'From there' is not necessarily the anchor point for the distance of the gym.
For example: From there(i.e. tennis club) on he went to gym 8 kms away. / From there on he went to gym 8 kms away (from his home).

Either way the answer can be 23 or 7 depending upon his direction of motion

I assume that here, maximum is 23 (15+8 =23) and minimum is 7( when 15-8 =7). but the question only stipulates the distance to be a straight line. for example 15 could be straight horizontal and then 8 could be perpendicular to that line, then we have 17 from home. But the 8km could be almost 180 (example 170) back towards the home then it's not 7 but little more than 7 or could be even 8.

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