First, we need to know how much she earns after the rise
15.50*.1=1.55
She earns after the rise
15.5+1.55=17.05
Then we need to know how much she earns per week
17.05*20=341
then we calculate the 20%
341*.20=68.2
each week she saves $68.2
after 4 weeks she save
68.2 *4=$272.8