If the person can't accommodate and the glasses is + 2.50d, at which distance will the person see clearly

At  2/5 of a Meter is where the person can see clearly.

At a + diopter measurement is a nearsighted person take the value of 2.5 and make it the denominator with 1 as the numerator to find the distance in meters that they can see clearly past their face. 
1/2.5 = 2/5 of a meter.