Being in Jack's bedroom is sufficient to be in Jack's house.

→ Not necessary as you can be in the kitchen and still be in Jack's house.

Being in Jack's house is necessary to be in his bedroom.

→ Not sufficient (aka necessary) to be in Jack's house to be in his bedroom.

Hope this helps :) Let's get that 170+

Same. I started using it and bought it long before it became subscription-based. Have lifetime free access now :)

Interested as well :)