Since  "the cost of a packet of cookies is 2/5 the cost of a packet of nuts", 

we can think that one packet of cookies costs 2x, while one packet of nuts costs 5x.


Then the next statement  "Nic used 2/7 of his money to buy 5 packets of cookies and 12 packets of nuts"

allows us to write this equation


    5*(2x) + 12*(5x) = ,      (1)


where M is the Nic's total money.


Simplify equation (1)


    10x + 60x =         (2)

       70x    =         (3)

      35*(2x) = .       (4)


From the last equation (4),  you see that Nic can buy 35 packets of cookies for the same money  

instead of 5 packets of cookies and 12 packets of nuts.   It gives the  ANSWER  to question (a).



Now, regarding packets of nuts of the question (b), we can deduce from equation (3)
    
    14*(5x) =          (5)


which means that for  of his money Nic can buy 14 packets of nuts.


It makes clear, that for   of his money, Nic can buy   = 7*3 = 21 packet of nuts.


It is the  ANSWER  to question (b).