The operations you would use are multiplication and subrtraction
(the reason for this is because you need to find the cost for 30 people normally --12*30 -- and then subtract the discount -- 3*30 -- to find the total cost)
In the mathematical sentence (also known as the equation) we'll let the variable 'c' equal the total cost for all 30 students.
This means c=12(30) -3(30)
To break it down you write it this way because you need to find the cost before the discount (12*30), then subtract the discount (3*30) to get the total cost (c).
Next, you just need to solve;
c=12(30)-3(30)
c=360-90
c=270
This means that the total cost for all 30 students will be $270.
I hope this helps!!!
