Mary carried 4 + 3 = <<4+3=7>>7 buckets of sand.
Iris carried 7 – 1 = <<7-1=6>>6 buckets of sand.
All together, they carried 4 + 7 + 6 = <<4+7+6=17>>17 buckets of sand.
The sand weighed a total of 17 * 2 = <<17*2=34>>34 pounds.
#### 34