At best buy they have a 42" sells for $1250 and is on sale for 15% off and the sales tax is 6.5%. what is the final cost?

The starting price is $1,250. We take off 15%. Convert the percent into a decimal. To do this we move the percent sign two places to the left.
15% => 0.15

Then we multiple: 1250 * 0.15 = 187.5
Subtract 187.5 from 1250. 1250 - 187.5 = 1062.5
Now we have to add sales tax. Which is 6.5%. Convert the percent into a decimal. To do this we move the decimal two places to the left.
6.5% => 0.065

Multiple: 1062.5 * 0.065 = 69.0625

Then add 69.0625 to 1062.5. 1062.5 + 69.0625 = 1131.5625
After round it to the nearest tenth, the final cost is $1,131.6

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