Hens and roosters

Ms. Mae raises chicken in her garden. She has 12 chickens, but there are 4 more hens than roosters. How many hens does she have?

Sissi and Aiden ate 20 cookies last night. "We each ate 10!" Sissi said. "No, I ate 2 more than you!", Aiden corrected her. "Oh yeah, I forgot I left one in the plate." Sissi agreed. How many cookies did Aiden eat last night?

Aiden and Sissi took turns working on Ms. Mae's computer to build a Scratch project. Together they worked for 15 minutes, but since Sissi had to go to the bathroom, Aiden worked 3 more minutes than Sissi. How many minutes did each of them work on the project?

Sissi and Aiden went to an orchard on Saturday. They picked 100 apples together, but Sissi worked longer so she picked 20 more apples than Aiden. How many apples did Aiden pick?

If x + y = 13 and x - y = 5, what is x and what is y?