Ineed $450 to buy a bike. i only have $72. what percent of the cost of the bike have so far?

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

Next, you would cross multiply. 
450x = 7,200

Now, all you would have to do is isolate the x. To do this, you would divide both sides by 450. 
x = 16%

You have 16% of the bike's cost. 

I hope this helps!