When I ran it, I didn't get an index error, but I ended up producing a = [2, 4, 6, 8]. One problem could be that as soon as you start deleting elements of a, moving left to right, its index will shift, and will no longer align with that of b. I tried counting backwards instead (and I skipped on using enumerate, as I'm a noob and I find it hard to remember how it works):
for i in range(len(b)-1,-1,-1):
if b[i] < 15:
del(a[i])
