The person is saving $20 per week so let w= number of weeks then the question asked how much needs to be saved to reach exactly 100 so you would use an equal sign. So the end result is 20w=100
You can also say that the amount of money going up by 20 and the weeks are going up by 1 so make the money side counted up to 100 first by using 20 and then go to week and go up by 1 till you get to the 100♂️
