Hello, 

Cost of 10 bananas = Rs. 45

Cost of 1 banana = 45/10

= Rs. 4.5

Selling price of 12 bananas = Rs. 51

Selling price of 1 banana = 51/12

= Rs. 4.25

Loss = Cost price of 1 banana - Selling price of 1 banana

Loss = 4.5 - 4.25

Loss = Rs. 0.25

Loss percent = (Loss*100)/Cost price

⇒ (0.25*100)/4.5

⇒ 25/4.5

⇒ 5.55%

So, he suffered a loss of 5.55 %.