cost of 16 big packets = cost of 40 small packets

ratio of big packets : small packets
= 16 : 40
= 2 : 5

So this means cost of 2 big = cost of 5 small

Since he already bought 6 big packets,
2 : 5
= 6 : 15

The cost of these 6 big packets is the same as cost of 15 small packets.

So total he already spent
= cost of 15 small packets + the 18 small packets he also bought
= cost of 33 packets

Now, we know he could buy 40 small packets with all his money.

So 40 - 33 = 7

He still has enough money to buy 7 small packets.