Last modified on October 26th, 2024

chapter outline

 

Cross Multiplying with Fractions

To cross-multiply fractions, we multiply the numerator of the first fraction by the denominator of the second fraction and vice versa.

In the algebraic equation ab=cd, the cross multiplication is given by the formula a×d=b×c

How to Cross Multiply a Fraction

Cross multiplication is used to compare fractions, add or subtract unlike fractions, find an unknown value in an expression, and compare ratios.

For example,

Cross Multiplying with Fractions

Thus, by cross-multiplying 35=610, we get 3×10=5×6 (which implies 30 = 30)

Cross-multiplication is also used to solve the value of the unknown variables in any equation. 

With One Variable

If we have one variable on each side, we solve the equation in the following way.

Let us solve 65=x20 for x

Cross-Multiplying

6×20=5×x

120=5x

Dividing both sides by 5

5x5=1205

x=24

Thus, the solution is x = 24

Problem – Cross Multiplying With NEGATIVE FRACTION 

Solve: x12=8448

Solution:

Given, x12=8448
By cross-multiplying, we get
x×48=12×(84)
48x=1008
On dividing both sides by 48,
48x48=100848
x=21
Thus, the solution is x = -21

With Two Variables

If the same variable is present on both sides of the equation, then we cross-multiply to find the variable.

Let us solve x9=4x for x

Cross-Multiplying

x×x=9×4

x2=36

Finding Square Roots of Both Sides

x=36

x=6

Thus, the solution is x = 6

Comparing

Unlike Fractions

To compare unlike fractions, we convert their denominators to a common value by multiplying the two original denominators together, ensuring both fractions have the same denominator.

For example, 

Let us compare 38 and 45 by cross multiplication.

Here, the denominators are 8 and 5

Thus, the denominators of both fractions become 8×5=40

Now, cross -multiplying to get the numerators,

3×5=15 and 8×4=32

Thus, the fractions become 1540 and 3240

Now, comparing the fractions,

1540 < 3240

38 < 45

When the resulting fractions are the same, they are equivalent fractions. 

Ratios

If two ratios are equal (where b, d ≠ 0), then the cross-multiplication product is also equal. 

On comparing the ratios: 

  • ab=cd, if a×d=b×c
  • ab<cd, if a×d<b×c
  • ab>cd, if a×d>b×c

Now, let us compare 2:4 and 5:10

Here, the fractions are 24 and 510

2×10=20 and 4×5=20

Since 2×4=5×10

Thus, the ratios 2:4 and 5:10 are equivalent.

If 12 cookies cost $60, what would be the cost of 5 cookies?

Solution:

Given, 
The cost of 12 cookies = 60
The cost of 1 cookie = 6012 …..(i)
Let the cost of 5 such cookies be x
The cost of 1 cookies = x5 …..(ii)
From (i) and (ii),
6012=x5
60×5=12×x
300=12x
On dividing both sides by 12, 
12x12=30012
x=25
Thus, 5 cookies cost $25

Last modified on October 26th, 2024