Let's assume the man had $100.
He buys the horse for $60. Hence he has $100-$60=$40.
Now he sell the horse for $70. So, now he has $40+$70=$110.
Again he buys the horse for $80. So, his remaining amount is $110-$80=$30.
Finally he sells the horse for $90. Hence, his final amount is $30+$90=$120.

Therefore subtracting his initial amount with final amount, we get $120-$100=$20 profit.