She is asaving money to buy a phone worth $1000. She already has $200. She can only save $20 per week. The number of week it will take her to buy the phone can be calculated below
Let
x = number of weeks
[tex]\begin{gathered} 1000=200+20x \\ 1000-200=20x \\ 800=20x \\ \text{divide both sides by 20} \\ x=\frac{800}{20} \\ x=40\text{ weeks} \end{gathered}[/tex]