Best answer will be mark as
the average weight of the seven dogs at the vet's office was 40 pounds. six of the weights were recorded: 71, 34, 15, 23, 18, and 56. what was the seventh weight of the dogs at the vet?

First of all,
The total weight of the six dogs are:
71+34+15+23+18+56=217 pounds
Then, we know that the average weight of the seven dogs are 40 pounds, so the total weights of the seven dogs are:
40×7=280 pounds.
Lastly, we know that the six dogs weights are 217 pounds and the total weight of the seven dogs are 280 dogs, so the seventh weight of the dog is:
280-217=63 pounds. As a result, the seventh weight of the dogs at the vet is 63 pounds. Hope it help!