One number is eight more than twice another. if their sum is decreased by ten. the result is thirteen. find the numbers

One number- we will call it x
It is eight 'more' so we'll add 
another- a different variable, we will call it y
It is eight more than 'twice' another so we will multiply 2 to the y variable
Our equation will then be: x=2y+8
Since x is 8 'more' than twice the other number
Now the sum of the two numbers are decreased (subtracted) by ten and the difference is thirteen.
x+y-10=13
Now add ten to both sides
x+y-10=13
     +10  +10
x+y=23
Now we know that the sum of the two numbers is 13.
Think of any reasonable number for y and we will continue from there. 
For sake of saving space, lets use 5.
(Keep in mind we must have both numbers add to equal 23.)
x=2(5)+8
x=10+8
x=18
y=5          18+5=23
That's the values :) Hope it helped