If you double a number and then add 20, you get 4/7 of the original number. What is the original number?

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!