Of course you can calculate that.
It's a question of combinations (and their percentual chances)... C(n,r) = n!/r!(n-r)!
If the chance to find a decoration is 5%, then the chance of not finding a decoration is 95%.
Let's list them from top down:
The chance to find exactly 0 decorations on 8 tries is:
C(8,0) × 5%^0 × 95%^8 = 1 × 1 × 66.342% = 16,983,563,041/25,600,000,000 = 66.34204%
The chance to find exactly 1 decoration on 8 tries is:
C(8,1) × 5%^1 × 95%^7 = 8 × 1/20 × 69.833% = 7,150,973,912/25,600,000,000 = 27.93349%
The chance to find exactly 2 decorations on 8 tries is:
C(8,2) × 5%^2 × 95%^6 = 28 × 1/400 × 73.509% = 1,317,284,668/25,600,000,000 = 5.14564%
The chance to find exactly 3 decorations on 8 tries is:
C(8,3) × 5%^3 × 95%^5 = 56 × 1/8000 × 77.378% = 138,661,544/25,600,000,000 = 0.54165%
The chance to find exactly 4 decorations on 8 tries is:
C(8,4) × 5%^4 × 95%^4 = 70 × 1/160000 × 81.451% = 9,122,470/25,600,000,000 = 0.03563%
The chance to find exactly 5 decorations on 8 tries is:
C(8,5) × 5%^5 × 95%^3 = 56 × 1/3200000 × 85.738% = 384,104/25,600,000,000 = 0.00150%
The chance to find exactly 6 decorations on 8 tries is:
C(8,6) × 5%^6 × 95%^2 = 28 × 1/64000000 × 90.25% = 10,108/25,600,000,000 = 0.00004%
The chance to find exactly 7 decorations on 8 tries is:
C(8,7) × 5%^7 × 95%^1 = 8 × 1/1280000000 × 95% = 152/25,600,000,000
The chance to find exactly 8 decorations on 8 tries is:
C(8,8) × 5%^8 × 95%^0 = 1 × 1/25600000000 × 1 = 1/25,600,000,000
The chance to find 5 (or more) decorations on 8 tries would be:
(384,104 + 10,108 + 152 + 1)/25,600,000,000 = 394,365/25,600,000,000 = 0,00154%
...or about one chance in 64,914.