Question 115685
I WILL GIVE YOU ONE OF EACH AND YOU CAN DO THE REST. ALSO LOOK UNDER WORD PROBLEMS ON THIS SITE AND YOU SHOULD FIND LOTS.

(1).  Johnny has a total of 21 apples and oranges.  He has twice as many apples as oranges.  How many apples and how many oranges does Johnny have?

Let x=number of oranges
And let y=number of apples

Now we are told that:

x+y=21-------------------eq1
y=2x--------------eq2

Substitute y=2x from eq2 into eq1

x+2x=21  collect like terms

3x=21  divide both sides by 3
x=7--------------------------number of oranges
from eq2, y=2x; y=2*7=14------------------number of apples

(2)  The sum of two numbers is 40 and their difference is 10.  Find the numbers.

Let x=one number
And let y=the other number

Now we are told that:
x+y=40--------------------------eq1
x-y=10--------------------------eq2

Add eq1 and eq2 and by doing so we eliminate y
2x=50  divide both sides by 2
x=25----------------------------------------------one number
substitute x=25 into eq1:
25+y=40 subtract 25 from both sides

25-25+y=40-25  collect like terms

y=15-------------------------------------------the other number

CK
25-15=10
25+15=40

Hope this helps--------------------------ptaylor