Since a year is 52 weeks, and Mr. Robert only saves one dollar a week, he does not have enough money to buy the 210 dollars penny because he will save 52 dollars which is not enough to buy it.
In order to buy that penny, he need to save more so:
210/52
=4.038
Therefore Mr. Robert will have enough money to buy a penny in about four years.
I really hope this answer helped you.
Have a wonderful day! ^_^