Let the radius of wheel = r cm

Now distance travelled wheel in one revolution = 11km/6250

                                                                    = (11*1000*100)/6250 cm

                                                                    = (11*1000*10)/625 cm      (100 and 6250 is divided by 10)

                                                                    = (11*1000*2)/125 cm        (10 and 625 is divided by 5)

                                                                    =  (11*40*2)/5 cm              (1000 and 125 is divided by 25)

                                                                    = (11*8*2) cm                    (40 and 5 is divided by 5)

                                                                    = 11*16 cm

Since distance travelled by wheel in one revolution is equal to the circumference of the wheel

So circumference of the wheel = 11*16

=> 2*π*r = 11*16

=> 2*22/7*r = 11*16        (π = 22/7)

=> 2*22*r = 11*16*7

=> r = (11*16*7)/(2*22)

=> r = (16*7)/(2*2)          (11 and 22 is divided by 11)

=> r = (16*7)/4               

=> r = 4*7                       (16 and 4 is divided by 4)

=> r = 28 cm

Now diameter of wheel = 2* radius of wheel

                                = 2*28                (radius of wheel r =28)

                                = 56 cm

So diameter of wheel = 56 cm