he sold 20 skateboards on wednesday and 10 skateboards on thursday so total was 30 skateboards.
from 12 pm to 6 pm is 6 hours. so he was selling them for a total of 6 hours on thursday.
in 3 hours he sells 5. 3 hours plus another 3 hours is 6 hours so for the 6 hours he worked he sold 10. ( 3 hours + 3 hours = 5+5 so 10 total.
