# Thread: Simple algerbra problem

1. ## Simple algerbra problem

Hi everyone,
i know i am way out of my league being on this forum but i am a mature student looking to get into college & for the corse i want i ned to repeat my maths exams! I tried signing up to a class but the pace was too fast for me. Anyway any help with this simple problem would be appreciated, thanks in advance!
Find the missing number in the following calculation:
-5x3x?=30
My question is this,
what is the formula or procedure to finding out the missing number, other examples are
(7?-5)x3=-9
or
?-(-3)x(-2)=-4
thanks again and i do apoligise for asking such a stupid question on here!
Laura

2. ## Re: Simple algerbra problem

You can study this guide.

3. ## Re: Simple algerbra problem

Traditionally in algebra, we represent the unknown quantity with a letter such as x or y, but any symbol will do, such as a question mark.

In the first example, we are given:

$-5\times3\times?=30$

The associative and commutative properties of multiplication say that we may move/group factors as we choose, .i.e:

$a\times b\times c=(a\times b)\times c=a\times(b\times c)=a\times c\times b$, etc.

So, we may write the equation as:

$(-5\times3)\times?=30$

Now, compute the product within the parentheses:

$-15\times?=30$

Since the unknown quantity is being multiplied by -15, we want to divide both sides by -15 to isolate the unknown on its side of the equation. Whatever we do to one side of the equation we must also do to the other.

$\frac{-15\times?}{-15}=\frac{30}{-15}=\frac{(-2)\times(-15)}{-15}$

Now, we may cancel or divide out factors common to the numerators and denominators, and we are left with:

$?=-2$

Second example: $(7?-5)\times3=-9$

Divide both sides by 3:

$\frac{(7?-5)\times3}{3}=\frac{-9}{3}=\frac{-3\times3}{3}$

$7?-5=-3$

Now we want to add 5 to both sides to isolate the term with the unknown factor ( $x+0=x$):

$7?-5+5=-3+5$

Simplify:

$7?=2$

Since the unknown is not separated by any characters, it is implied that we have the product $7\times?$.

Divide both sides by 7:

$\frac{7?}{7}=\frac{2}{7}$

$?=\frac{2}{7}$

Third example: $?-(-3)\times(-2)=-4$

Simplify by computing the product on the left side (a negative times a negative is positive, but this positive is being subtracted):

$?-6=-4$

Add 6 to both sides:

$?-6+6=-4+6$

$?=2$

4. ## Re: Simple algerbra problem

-5x3x?=30
In this question we find out the ? as:
?=30/(-5*3)
?=30/(-15)
?=-2

Now similarly, all the other questions of this category can be solved same:
(7?-5)x3=-9
(7?-5)=-9/3
(7?-5)=-3
(7?=-3+5
7?=2
?=2/7

Similarly,
?-(-3)x(-2)=-4
?-(-3)=-4/-2
?-(-3)=2
?=2/3

Thus you can see you can solve every question falling under this category is solved step by step & equalizing each term accordingly.

5. ## Re: Simple algerbra problem

Thanks so much for all of your replies I really appreciate it! I might be back for more help soon ( well might is a lie I will be back soon ;-)