Lets say he first buys n bananas at 6 .
So the cost = ( n/5 ) * 6 = 6n/5
He gets n again at 15 per dozen
=> Cost = 15n/12 = 5n/4
=> Total CP = 6n/5 + 5n/4 = 49n/20
He mixes the two and selles them at 14
=> Selling Price = ((n + n ) /12 ) * 14
= 14n/6 = 7n/3
Profit or loss = SP - CP = 7n/3 - 49n/20
= n ( 7/3 - 49/20 )
Do the rest ...
If it comes negative , then he incurred loss . If it comes positive , then it was a profit.
You can find the percentage by
( SP - CP ) / CP * 100