this is a sum on relative velocity.this sum becomes easier when use the relative velocity of the fast train with respect to the slow train

velocity of fast train=115 km/hr

velocity of slow train=25 km/hr

relative velocity of fast train with respect to the slow train=115-25

                                                                                 =90 km/hr

                                                                                 =90*(5/18)m/s

                                                                                 =25 m/s

now,initial velocity(u)=25 m/s,final velocity(v)=0 m/s,distance(s)=100 m

v²=u²+2as

0=(25*25)+2*100*a

-200a=625

-a=625/200

-a=3.125

a=-3.125m/s

the least retardation must be 3.125 m/s