There are a total of 63 bikes. if the the ratio of blue bikes to black bikes is 4 to 5, how many of the bikes are blue

I like to approach things like this like so:

First, add the four and five together. Now we know there will be nine parts.

Next, we divide 63 by 9, which is 7. Each part is 7 bikes.

Finally, we multiply 7 by 4, because blue is four parts of the whole. 

28 of the 63 bikes are blue.

Hope I helped!

Let x and y be the number of blue and black bikes, respectively.

We have x:y = 4:5
⇒ x= 4/5*y

We also have x+y=63 (because the number of bikes is 63)
⇒ 4/5*y + y = 63
⇒ (9/5)y = 63
⇒ y= 63/ (9/5)
⇒ y= 35

x = 63-35 = 28.

There are 28 blue bikes~