Before, you got 9 jars.

After, you left with 3 jars.

6 jars = 20 × 9 = 180

1 jar = 180 ÷ 6 =30 before

30 – 20 = 10 after

5 packet = 180

1 packet = 180 ÷ 5 = 36

36 – 10 = 26

Number of cookies in each jar at first -> u

Total number of cookies in 3 jars -> 3u

Number of cookies in each jar in the end -> u – 20

Total number of cookies in 9 jars -> 9(u – 20) = 9u – 180

9u – 180 = 3u

6u = 180

u = 30

Number of cookies in each jar in the end -> u – 20 = 10

Number of cookies in each packet -> 36  (found by your nephew)

Difference -> 36 – 10 = 26