a reciprocal is the inverse of a number. if you are given a whole number such as 2 or 3 or 4 or 5... etc. then you flip the number by putting a 1 as the numerator. so the reciprocal of 2 is 1/2 and for 3 it is 1/3 and 4 is 1/4... and so on...
first put your decimal in fraction form
2.5 = 2 and 5/10
= 2 and 1/2
= 5/2 ---> 2/5 (0.4) is the reciprocal of 2.5