I have the answer but I don't know why. I wrote down my calculating progress(please see the attached picture.)

Please tell me how to get to the answer 100% or my understanding is wrong. Thank you for helping.

At an ocean-side nuclear power plant, seawater is used as part of the cooling system. This raises the temperature of the water that is discharged back into the ocean. The amount that the water temperature is raised has a uniform distribution over the interval from 10 to 25 ^oC.

(f) What is the probability that the temperature increase will be within 2 Standard Deviation from the mean? 100%

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