泽维尔（Xavier）计划用他的零花钱存下 $21。下表显示了他周一至周五的每周零花钱储蓄计划。

星期	储蓄金额
周一	$50\text{c}$
周二	$50\text{c}$
周三	$60\text{c}$
周四	$60\text{c}$
周五	$80\text{c}$

泽维尔从一个星期五开始储蓄。他在星期几刚好存满 $21？

1. Calculate the total amount saved in one full week of saving (5 days).

$50 + 50 + 60 + 60 + 80 = 300\text{ c} = \char36{}3$

2. Find the number of full weeks needed to save exactly $21.

$\char36{}21 \div \char36{}3 = 7\text{ weeks}$

3. Determine the day of the week the 7th week ends on.

Since Xavier starts saving on a Friday, one full cycle of 5 saving days is Friday, Monday, Tuesday, Wednesday, and Thursday. The cycle ends on a Thursday. Since 7 weeks is an exact number of cycles, he will reach exactly $\char36{}21$ on a Thursday.