How would you go about this problem please help me.

Mrs Santaniello wants to buy lawn feed.
Sold in 2 sizes.

Packet size
2kg = £13.00
5kg = £28.50

35 grams is needed per sq metre.

Lawn is 208 square metres. She wants to apply it just once.

What packet sizes should she buy to get the cheapest price?

Explain answer.