one and two add/s

[Tan Elaine]

Once there was a boy who thought highly of himself. One day, while he and his parents were having dinner, he saw two chicken drumsticks on a plate on the dinner table.

He told his parents that there were three drumsticks. His mother told him that there were only two. He insisted that there were three. He pointed at the first drumstick and said, “This is one.” Then pointing to the second drumstick, he said, “That is two. So one and two add/s up to three.”

Should it be 'add' or 'adds'? Are there any errors in the text?

Thanks in advance.

 

[2006]

Once there was a boy who thought highly of himself. One day, while he and his parents were having dinner, he saw two chicken drumsticks on a plate on the dinner table.

He told his parents that there were three drumsticks. His mother told him that there were only two. He insisted that there were three. He pointed at the first drumstick and said, “This is one.” Then pointing to the second drumstick, he said, “That is two. So one and two add/s up to three.”

Should it be 'add' or 'adds'? Are there any errors in the text?

Thanks in advance.

'add' is correct. (The two numbers)(They) add up to three.