let cat food = x
total plus 5% = 105% = 1.05
1.05 * (11.50 + X) = 16.80
12.075 + 1.05x = 16.80
1.05x = 16.80-12.075
1.05x = 4.725
x = 4.725 / 1.05
x= 4.50
the cat food cost $4.50
11.50 +4.50 = 16.00
16.00 * 0.05 = 0.80
16.00 + 0.80 = 16.80
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