Answer:
Online store will save the most money
Step-by-step explanation:
<u>Amount used in Physical store</u>
Batteries    2packs      @$5.99     cost = $5.99*2=$11.98
Stereo        1 system     @$100      cost = $100*1=$100
CDs             3 CDs        @$7.95     cost = $ 23.85
Total cost= $11.98+$100+$23.85= $135.83
Applying the 10% off coupon = 90/100 * $135.83 =<u>$ 122.25</u>
Applying sale tax 5% = 105/100 * 122.25=<u>$128.36</u>
Amount  to spend when using physical store= $128.36
<u>Now</u><u> </u><u>calculate for the online store</u>
Batteries     2packs     @$4.99       cost= $4.99*2=$ 9.98
Stereo         1 system    @$90         cost= $ 90 *1= $90
CDs              3  CDs      @$8.95       cost = $8.95*3=$26.85
Total cost=$9.98+$90+$26.85=$126.83
Applying the 5% off coupon= 95/100*126.83=$120.49
Applying the sale tax, $6.99+$120.49=$127.48
Comparing the two ways, 
Physical store= $128.36
Online store=$127.48
Online store will save the most money by ($128.36-$127.48)=<u>0.88</u>