Let rate upstream  = `x` kmph and  rate  downstream = y kmph

Then , `24/x + 36/y` =  36 ..............(I)

and  `36/x + 24/y =  13/2 `................(II)

Adding (I) and (II) , we get  : 60`(1/x + 1/y) = 25/2` or `1/x + 1/y = 5/24` .................................(III)

Subtracting (I) from (II) , we get  : ` 12(1/x - 1/y) = 1/2` or `1/x - 1/y = 1/24 `................................(Iv)

Adding (III) and (IV), we  get  : `2/x = 6/24   or  x` = 8

So, `1/8 + 1/y = 5/24      hArr  1/y = (5/24 - 1/8) =  1/12         hArr  y `= 12.

Speed  upstream = 8 kmph , speed downstream = 12 kmph

Hence , rate of current = `1/2 (12 - 8)` kmph =  2 kmph.