If you would like to know how much did Tan spend on electricity over the three-month period, you can calculate this using the following steps:
August: $267
September: 2% * $267 = 2/100 * 267 = $5.34
$267 + $5.34 = $272.34
October: 8% * $267 = 8/100 * 267 = $21.36
$267 - $21.36 = $245.64
Now, you have to add this three numbers: $267 + $272.34 + $245.64 = $784.98
The result is none of the listed above, because the correct result is $784.98.