From the first picture, we are given 5 match sticks.
From the second picture, we are given 9 match sticks. The difference between the match sticks is 4.
From the third picture, we are given 13 match sticks. The difference between the match sticks is again 4.
If each image increases the number of match sticks by 4, then the fourth image must have (13 + 4) = 17 match sticks. The fifth image must have (17 + 4) = 21 match sticks. The sixth image must have (21 + 4) = 25 match sticks. The seventh image must have (25 + 4) = 29 match sticks. And the eighth image must have (29 + 4) = 33 match sticks
