Well for the activity we can do a game.
In a game you start with 50 coins and everytime you kill a monster "x" you get 2 more coins.You need 100 coins to level up.
We can make the following inequality,
2x + 50 ≥ 100
So to find x we single it out,
2x + 50 ≥ 100
-50 to both sides
2x ≥ 50
Divide 2 by both sides
x ≥ 25
For the number line look at the image below ↓
The solution in the number line is the number of monsters killed in order to level up.
Hope this helps :)