`u_i = (x_i-a)/h = d_i/h`
`d_i = x_i-a`
assumed mean`= a =18`
`h= 13-11 = 2`
acc to series
`x_i= 12,14,16,18,20,22,24`
`f_i = 7,6,9,13,f,5,4`
`d_i= -6,-4,-2,0,2,4,6`
`u_i= -3,-2,-1,0,1,2,3`
`f_iu_i= -21,-12,-9,0,f,10,12`
`sum f_i= 44-f`
`sum f_iu_i= f-2o`
mean`= a+ ((sum f_iu_i)/(sum f_i))h`
`18 = 18 + ((f-20)/(44+f))*2`
`(f-20)/(44+f) = 0`
`f-20=0`
`f=20`
answer