NOTE: Almost all subjects have different rules regarding rounding off and significant figures. I'll be using the rules that I usually apply in school.
Marcus:
$35.99 * 5.5% = $1.98
$1.98 + $35.99 = $37.97 (Total Cost)
Tia:
Since it wasn't specified if the coupon was applied before or after tax, we're going to assume that it was applied before tax because that's usually how it works.
$39.99 * 15% = $5.9985 or $6.00
$39.99 - $6.00 = $33.99
$33.99 * 9% = $3.0591 or $3.06
$33.99 + $3.06 = $37.05 (Total Cost)
Difference in Marcus and Tia's Total Cost:
$37.97 - $37.05 = 0.92 cents
