math
posted by TUHITUHI on .
Sanjeev is twice as old as his son.If after four years he will be four times as old as his son was 9 years ago ,how old is each now ??

Let's let J represent Sanjeev and x represent his son.
If J is twice as old as S then the equation is 2x=J
After four years, so J+4, he is four times as old as his son, so 4x, was 9 years ago, so x9, if you combine all the parts of that statement you get the equation: 4(x9)= J+4.
So now you have two different equations:
2x=J
4(x9)=J+4
So now you just substitute. We know that J=2x from the first equation so substitute 2x in for J in the second equation and solve for x:
4(x9)=(2x)+4
4x36=2x+4
2x36=4
2x=40
x=20
so now we know that the son is 20 years old. Then plug in 20 for x in the first equation to find Sanjeev's age:
2(20)=J
40=J
So Sanjeev is 40