Apair of jeans originally costs $45.00. if the jeans are on sale for 30% off, what is the new cost of the jeans?

You can solve this using percent proportions.
The equation is part/whole =x/100
the equation for percent proportions in this context would be x/45=30/100 because we already know the percentage.
We then cross multiply, getting 13.50
Now 13.50 isn't the new cost, it's the discount. You then would subtract 45 by 13.5 to get 31.50
The jeans now cost $31.50

Alternatively, you could calculate that 10% of $45 is 4.50, then multiply that by 3. 4.5*3=13.5, then subtract that from the original price of 45. Less steps, just as effective.