Let the man purchase 6k oranges.
==> CP of 6k oranges = Cost of one orange x No. of oranges bought
= (5/2)x6k = Rs.15k
SP of 6k oranges = SP of one orange x no. of oranges sold
= (8/3) x 6k = 16k.
Therefore Profit = SP - CP = 16k -15k = 1k = Rs.20 (given)
==> k = 20.
Hence man purchased oranges = 6k = 6x20 = 120