Let's write down what you said:
you double a number
2x
and then add 20,
2x+20
you get 4/7 of the original number.
2x+20=4/7x.
we can get rid of the fractionby multiplying everything by 7:
14x+140=4x
and subtract 4x from both sides:
10x+140=0
move 120 to the other side:
10x=-140
and divide by 10
x=-14
and that's the answer!
