I don't know what method they want you to use to find this, but you could just keep adding 3 to the previous number. Week 5 she has 25. Week 6 she has 28. Week 7 she has 31. 8, 34. 9, 37. 10, 40. 11, 43. 12, 46. 13, 49. 14, 52. 15, 55. 16, 58. 17, 61. And finally, at week 18, she will have 64 stamps.