Maths: How do you work out ratios

I had a maths test and one of the questions was: windscreens have a ratio of 9:16 if you had to draw the screen and the height was 4.5 centimetres how wide would the screen be? Can someone give me the answer and an explanation on how to work it out? Also there was another question that was if the ratio was 3:4 and the height was 4.5 cm how wide would that screen be?

Ratios can also be written as fractions.

9/16 = 4.5/x

When you have two fractions like this, you can CROSS-MULTIPLY. This means
a/b = c/d
is the same thing as
a * d = b * c

So
9/16 = 4.5/x
Is the same as
9x = 16* 4.5
9x = 72
x = 72/9
x = 8
The screen would be 8 cm wide (not much of a windscreen ;) )

Same for the other one:
3/4 = 4.5/y
is the same as
3y = 4 * 4.5
3y = 18
y = 18/3
y = 6

Like that :)

Height : Width
9 : 16
If height is 4.5cm that is 1/2 of 9, then Width will be 1/2 of 16, that is 8.
4.5 : 8
So the screen would be 8 centimetres wide.
For the next question:
Height : Width
3 : 4
If height is 4.5 cm that is 3/2 of 3 .....how?
4.5 = x of 3
4.5 = x(3)
x= 4.5/3 = 3/2
then Width will be 3/2 of 4,that is 6.
So the screen would be 6 centimetres wide.

Q1
ratio ... 9:16 ....... you did not mention this ratio is height to width or width to height?

suppose it is height to width then
9 is to 16 as 4.5 is to width
9/16 = 4.5/W .... 9 W = 4.5 * 16 ..... W = 8 centimeter

Q.2
ratio ... 3:4 ....... you did not mention this ratio is height to width or width to height?

suppose it is height to width then
3 is to 4 as 4.5 is to width
3/4 = 4.5/W .... 3 W = 4.5 * 4 ..... W = 8 centimeter

...........

Ratios to actual are 9:16::4.5: x
Redraught this as
9/4.5 = 16/x
Algebraically rearrange
x = 16 x 4.5 / 9 = 16 x 1/2 = 8
x = 8 (The width)

Similarly
Ratio to actual are 3:4 :: 4.5: x
3/4,5 = 4/x
x = 4 x 4.5 / 3 = 18/3 = 6
x = 6 ( the width).

Hope that helps !!!!

this are always done with CROSS MULTIPLICATION

if
a......c
-- = --
b.....d

then ad= bc

so
9/16 = 4.5/x
so 9x = 16*4.5
9x=72
x=8

dear, im also asking the same question...since I DONT FREAKING KNOW....