Every 5 packets cost $1.85
John can afford 9.25 : 1.85 = 5 (times)
So, John bought 5 x 5 = 25 packets of noodles.
If he were to buy 40 packets, he would've had to pay 40 : 25 = 1.6 times more money.
So, he would've had to pay : 9.25 x 1.6 = 14.8 (dollars)
Since 14.8 < 15, $15 would be enough to pay for 40 packets of instant noodles.