78% of 380 round to the nearest thenth if necessary

So I learned this really cool way of finding percents of numbers in my 7th grade math class, hopefully this will help! 
In most percent word problems it will have specific wording. For example: What is 25% of 100. Or 75 is 30% of what number? Those are examples of some of the wording (and random numbers) those word problems might use.
The trick is:  

so if i have a word problem that says: what IS 15% of 600? here is how your equation would look.  
Because you don't know what the "is" is, then you would put x there. If it say 40 is 17% of what? than it would look like this... 
To solve just cross multiply or use basic algebra to solve. Using basic algrbra on that last problem I just wrote out would look like this: