Q&A for How to Cross Multiply

Multiply 65 by 0.3. (You could think of it as multiplying 65 by 10% by just moving the decimal point to the left one digit to get 6.5 and then multiplying by 3 to get 30% of 65.)

Add together the two numbers in the ratio: 5 + 11 = 16. Divide that sum into the total number of visitors: 176 ÷ 16 = 11. Multiply that quotient by the ratio number representing the non-members (11): (11)(11) = 121 non-members. (If you wanted to find the number of members, you'd multiply 11 by the 5 in the ratio: (11)(5) = 55 members.)

The above technique works with all fractions. Some fractions are just more complicated than others, as in Method 2 above.

Multiply the whole number by the numerator, and keep the same denominator. Reduce the fraction if possible. (Another way of looking at it is to convert the 8 to the fraction 8/1, then create a new fraction by multiplying the two numerators together to get the new numerator, and multiplying the two denominators together to get the new denominator, which will be the same as the old denominator. Reduce if possible.)

Cross-multiplying works only when one fraction is set equal to the other. In other words, the "equals" sign must appear between the two fractions.

Multiply 145 by 0.6.

Multiply both sides of the equation by 6, because 6 is the lowest common denominator of the equation (because 2 times 3 is 6). That turns the equation into this: 3x - 4 = 4x. Subtract 3x from both sides of the equation, so that -4 = x.

First reduce 19/38 to 1/2. So 1/2 = x/19. Then cross-multiply: (1)(19) = (2)(x). So 19 = 2x, and x = 19/2, or 9½.

Multiply the three numerators together to get the final numerator. Do the same with the denominators. Reduce if necessary.

(k + 1) / (k - 12) = 4/3. Cross-multiply: (k + 1)(3) = (k - 12)(4). So (3k + 3) = (4k - 48). Subtract 3k from both sides, and add 48 to both sides: 51 = k.

In cross-multiplying, the signs affect the sign of the product as they would in any multiplication.

Do it the same way you do with integers.

25.5 ÷ 50 = 0.51 oil.

Cross-multiplying is appropriate only when one fraction (or ratio) is set equal to another. If we assume that the equation is 2 - x + (2/5x) = 3/5, that's not one fraction equal to another. Instead you multiply both sides of the equation by the LCD, 5x. That will clear all fractions and make the equation fairly easy to solve.

Cross multiplication only works with two fractions on either side of an equals sign.

Cross multiplying doesn't work when you are adding two fractions. It only works when you have two fractions on either side of an equality (or inequality).

Cross-multiplying doesn't work when you're using units (like dollars or ounces). It applies only when you're using numbers without units.

See Understand the Basics of Matrices .

Cross multiply: (17)(165) = (55)(x). 2805 = 55x. Divide both sides by 55: x = 51.

You don't. It's simply a very fast way to solve equations that involve fractions.

You can solve for any variable in terms of the other variables. For example, if a/b = c/d, by cross-multiplying we find that ad = bc. Then a = bc / d, d = bc / a, b = ad / c, and c = ad / b.

Start by multiplying the numerators by the opposite denominators to get the equation 15x = 14x - 28. Then, combine like terms by subtracting 14x from both sides to get x = -28. There's your answer.

First you multiply the right numerator by the left denominator to get one product. Then you multiply the left numerator by the right denominator to get a second product. Then you set the first product equal to the second product and solve that equation for the unknown term.

Sine is the ratio of the opposite side to the hypotenuse (of a right triangle). Cosine is the ratio of the adjacent side to the hypotenuse. Tangent is the ratio of the opposite side to the adjacent side. It's not the angle that's adjacent or opposite; it's the side.

Some day you may need to take a particular ratio and apply it to a real-life situation (perhaps with a recipe or other formula). Cross multiplication helps you do that.

First cross-multiply: (m + 4)(m - 2) = (10)(4). So m² + 2m - 8 = 40. Then m² +2m - 48 = 0 = (m + 8)(m - 6). That means m = -8 or 6. Either solution works.

If you're trying to find an equivalent fraction, cross-multiplying is helpful. For example, if you're trying to find a fraction that is equivalent to 2/3 and has a denominator of 18, set up the equation 2 / 3 = x / 18. Cross-multiply: (2)(18 = (3)(x). So 3x = (2)(18) = 36, and x = 12. Thus 2/3 = 12/18.

Gently remind your parents that you're not trying to upset them or make them mad. Tell them you really want to do well at math but that you find it difficult. Suggest the possibility of hiring a tutor for you, someone with experience at explaining math concepts.

The third term, 2, has only two factors, 2 and 1. The coefficient of the middle term, 3x, is the sum of 2 + 1, so the factors of x² + 3x + 2 are (x + 2) and (x + 1).

Subtract 2x from both sides of the equation, and then add 4/3 to both sides. That leaves x = 13/12.