The answer is that it cannot be modeled with a rectangle.
$30 + $35 = $65
Assuming tax is on whole sale not each item:
$65 * 0.06 (6%) = $3.90 (6% of $65)
$65 + $3.90 = $68.90 ($65 + 6%)
OR
You could combine both of these steps and just use this working:
$65 * 1.06 (106%) = $68.90 (106% of $65)
You can use the break apart strategy which would look like:
368+231= 300+60+8 + 200+30+1
Or you could just plain and simple add 368+231
42% ................................
