Answer:
24
Explanation:
because in first piggy bank has 4 coins and second has 6 . difference=6-4=2
second has 6 coins and third has 9 . difference= 9 - 6 = 3 third has 9 coins and fourth has 13 . difference=13-9=4
fourth has 13 and fifth has 18 coins difference is 18 -13 =5
then,sixth has 24 because all have 2 3 4 5 difference then six has 6