(Grade 10 Math) Find the unknown number

[u/Granger0100]

How do I solve this? Like, what steps do I take? The decimal points are confusing me a bit.

[u/fermat9990]

If a/x=b/c then

bx=ac and

x=ac/b

[u/Serafim91 → u/fermat9990]

Psychopath! Who writes bx=ac instead of ac=bx?

[u/fermat9990 → u/Serafim91]

Who writes bx=ac instead of ac=bx?

😀 The ancient Greeks: The product of the means equals the product of the extremes

[u/[deleted] → u/fermat9990]

Way too complicated for this imo.

6.4/4 = 1.6

[u/Sabreblade11 → u/[deleted]]

Give a person an answer and they'll have the right answer for that question.
Teach a person how to arrive at the answer and they'll have the right answer to all similar questions.

[u/Teagana999 → u/fermat9990]

Precisely. The decimals are there to confuse you, but they don't make the process any different.

[u/PartyFuture142 → u/fermat9990]

0.9/x = 3.6/6.4 3.6x = (0.9)(6.4) 3.6x = 5.76 3.6x/3.6 = 5.76/3.6 x = 1.6

[u/MW1369]

Cross multiply that shit

[u/TooTToRyBoY]

Do not let the decimals scare you! Try to simplify the right side as you do with normal fractions.

[u/KevinbeParker → u/TooTToRyBoY]

Don't worry about the decimals at all, multiply both numerator and denominator by 10 on both sides of the equation. And you get to ignore them entirely (at least for a little while).

[u/[deleted]]

If decimals scare you here, multiply everything by 10. 9 / 10x = 36 / 64

There are a lot of ways to solve this. You're able to use the butterfly method (which is multiplying the inverse side), giving you (9*64) = 36x or 576 = 36x, which equals 16. Since we multiplied the equation by 10, we divide by 10 for an answer of 1.6.

You could also solve the equation algebraically because it's an inequality. The way you handle a fraction in algebra is to multiply both sides by the denominator that you want to get rid of. In this case, we want to 'get rid of' x. For simplicity I'll use the original numbers:

(.9 / x) = (3.6 / 6.4)
(.9 / x)(x) = (3.6 / 6.4)(x)

Multiplying the fraction by x removes it completely (If you don't understand why, substitute numbers into x), giving us:

.9 = (3.6 / 6.4)x

Simplifying the right side gives us

.9 = .5625x
.9 / .5625 = 1.6

x = 1.6

[u/Commercial_Key_1972]

my 6th grade brother can do this 💀😭

[u/qwertyjgly → u/Commercial_Key_1972]

this isn’t the place to be insulting people based on their skills. if you can’t be nice then kindly don’t say anything 🥰

[u/Birddman124 → u/Commercial_Key_1972]

And I know many many many adults in the working world who couldn’t do this

[u/peckx063]

3.6 is 4 times the size of 0.9

6.4 is 4 times the size of...?

1.6

[u/chicken_tendees7]

if you’re stuck on this in 10th grade, you need to get off reddit and get studying lmao

[u/Azylim]

Simple rule is whatever you do on the right side you have to do on the left side, and you want to isolate X (not 1/x)

0.9/x = 3.6/6.4

multiply both sides by x to get x out of the denominator

0.9 = (3.6/6.4)(x)

divide both sides by 3.6/6.4 to isolate x.

(0.9)(6.4/3.6) = x

A quick rule you can also use is that

0.9/x = 3.6/6.4

can be quickly switched into

x/0.9 = 6.4/3.6

which is easier to work with

[u/lll_Death_lll]

y/x=z
y/z=x
x=0.9/(3.6/6.4)
x=(9/10)/((36/10)/(64/10))
x=(9/10)/(36/64)
x=(9*64)/(10*36)
x=(1*64)/(10*4)
x=16/10
x=1.6

OR

(a/b)=(c/d)
(a*c)=(b*d)
0.9*6.4=x*3.6
x=(0.9*6.4)/3.6
x=9*64/360
x=64/40
x=16/10
x=1.6

[u/cmacfarland64]

This is not 10th grade math. This is like 5th grade math. Cross multiply yo.

[u/melinex01]

If you notice that 3.6 is 0.9 x 4, then x must be 1/4th of 6.4 which is 1.6

[u/DarkStar0129]

Cross multiply and go from there

[u/PoliteCanadian2]

Since each side is a fraction you can cross multiply.

[u/BrickBuster11]

Super Easy:

Step 1 invert both fractions, X/0.9=6.4/3.6

Step 2 Isolate X. X=(6.4*0.9)/3.6

Step 3 calculate, X=1.476.....

[u/Pototototo]

Simplify the right hand side.

3.6 / 6.4 when you divide the top and bottom by 2 gives you 1.8 / 3.2.

Do the same step again and you get 1.8 / 3.2 = 0.9 / 1.6

x = 1.6

[u/JustALittleInkyBoy_]

E : M = M : E -> M= (E • E) / M -> E= (M • M) / E

In this case: 0.9 : x = 3.6 : 6.4 -> x= (0.9 • 6.4) / 3.6