A rectangle has 2 sets of equal parallel lines. And we know one of the sides is 15.
So we know two of the sides have to be 15. Right?
So that means 15 x 2 = 30
So we know that two of the sides add up to equal 30.
The entire perimeter is 90. Well we've used 30 so lets see how much we have left to use.
90(the entire perimeter) - 30(the amount we know) = 60(what we have left)
So the last two sides of the rectangle equal 60, since there are two sides, let's divide the remaining perimeter by 2
60(both of the sides) / 2(the last 2 sides that we haven't found the length for yet) = 30
So now we know all of the sides.
Let's read the question again now.
"Find the 'WIDTH' of a rectangle with a perimeter of 90 and a 'LENGTH' of 15"
So it's 15 long(length), and 30 wide(width)
If this doesn't help, I can draw a picture and it mights make some more sense :)
