Out of 30 questions, ahmad answer 12 of them incorrectly. what percent of the questions did he answer correctly?

First, you would need to determine how many questions he got correct. 
30 - 12 = 18

Next, you would need to create an equation for this problem. Let x represent the unknown percentage. 
=

Now, you would cross multiply. 
30x = 1,800

The last step would be to isolate the x. To do this, you would divide both sides of the equal sign by 30. 
x = 60

Ahmad got 60% of the questions correct. 

I hope this helps!

Ibelieve the total for the tip could be $21.90

Idon’t know how to do that