Bucket A has 10 more seeds than bucket B: so there are 10 seeds + 30 seeds = <<10+30=40>>40 seeds in bucket A.
Bucket A and bucket B have 40 seeds + 30 seeds = <<40+30=70>>70 seeds together.
If there are 100 seeds in all the buckets, then bucket C has 100 seeds - 70 seeds = <<100-70=30>>30 seeds.
#### 30