Apply tax, tip, or discount to the wrong subtotal or in the wrong order.
An error-pattern drill: practice spotting and avoiding this specific SHSAT trap.
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A meal costs 50 dollars. Tax is 8%, and you leave a 20% tip on the pre-tax amount. The tax is 4 dollars and the tip is 10 dollars, so the total is 64 dollars. If you tip on the 54-dollar total after tax, the tip is 10.80 dollars and the total is 64.80 dollars. That is a trap answer.
The same thing happens with sales. 20% off an 80-dollar item gives a sale price of 64 dollars. A 5% tax goes on the 64 dollars, so the total is 67.20 dollars. If you tax the original 80 dollars, you get 68 dollars.
Check habits:
Tax \(= \$78.00 \times 7/100 = \$5.46\); tip \(= \$78.00 \times 18/100 = \$14.04\) (both on the pre-tax amount); total \(= \$97.50\). (Figuring the tip on the after-tax amount (\(18\%\) of $83.46) gives about $98.48. Forgetting to add the tax gives $92.04. Forgetting to add the tip gives $83.46.)
Sale price \(= \$48.00 \times (1 - 44/100) = \$26.88\); tax \(= \$26.88 \times 8/100 = \$2.1504 \approx \$2.15\); total \(= \$29.03\). (Figuring the tax on the original price (8% of \(\$48.00\) is $3.84) gives $30.72. Forgetting to add the tax gives $26.88. Subtracting the tax instead of adding it gives about $24.73.)
The total is 105% of the sale price: \(84 \div 1.05 = 80\). The sale price is 80% of the original: \(80 \div 0.8 = 100\). The original price is \(\$100\).