# how do i turn word problems into algebra format

john is 4 years younger than his sister mary. the sum of their age is 20. using algebra by letting marys age = x and constructing an equation. find johns age. i dont know how to put this in algebra form. can you help. thanks

ok so all it's asking you to do is substitute words for letters
chloe
25 May 2011
ok so Mary's age is x
chloe
25 May 2011
sorry scrap that! we've got all the info in the question
chloe
25 May 2011
we're told to use x for Mary's age so let's use y for John's age
chloe
25 May 2011
the sum of their two ages is 20 and we can write this as (x-4) + x = 20 which is basically the same as saying John's age plus Mary's age equals 20
chloe
25 May 2011
John is 4 years younger than Mary so his age is x-4
chloe
25 May 2011
2x = 24
chloe
25 May 2011
2x-4 +4 = 20 + 4
chloe
25 May 2011
then the next step is to add 4 to both sides to get the 2x on it's own:
chloe
25 May 2011
2x -4 = 20
chloe
25 May 2011
to do this we have to simplify the equation a little to:
chloe
25 May 2011
so now we just have to work out what x is
chloe
25 May 2011
so we now know Mary's age = 12 and because we were told that John is 4 years younger we also know that his age is 12 -4 = 8
chloe
25 May 2011
then all we have to do is divide both sides by 2: x = 12
chloe
25 May 2011
sorry i still do not get where you got 2x=24. i am really confused
coolgirl
25 May 2011
we get that because we start from 2x -4 = 20 - in order to get rid of the 4 on the left hand side of the equation, we have to add a 4 to both sides of the equation - this gives us 2x = 24
chloe
25 May 2011