Most GED percent questions use one idea: part = percent × whole, with the percent written as a decimal. Percent change divides by the starting amount. Two discounts in a row do not add. Simple interest is I = Prt, with the rate as a decimal and time in years. Practise each one here with worked steps.
The percent relationships
Find the part: part = percent × whole. Example: 30% of 240 = 0.30 × 240 = 72.
Find the whole: whole = part ÷ percent. Example: 36 is 15% of what? 36 ÷ 0.15 = 240.
Find the percent: percent = part ÷ whole × 100. Example: 42 of 56 = 42 ÷ 56 = 0.75 = 75%.
Percent change: change ÷ old × 100. Example: $80 to $92: 12 ÷ 80 = 0.15 = 15% increase.
Why two discounts do not add
20% off and then another 10% off leaves 0.80 × 0.90 = 0.72 of the price, which is 28% off, not 30%. Take each discount from the latest price. Sales tax works the same way: find it on the sale price, not the original.
Simple interest on the GED
I = Prt is on the official formula sheet (I = interest, P = principal, r = rate, t = time). Write the rate as a decimal (6% = 0.06) and the time in years (18 months = 1.5 years). Example: $2,000 at 6% for 18 months earns 2,000 × 0.06 × 1.5 = $180.
What the lab does
Generated problems cover the part, the whole, the percent, percent change, discount then tax, successive discounts, unit prices, proportions and simple interest, each with hints, worked steps and feedback that names the likely mistake. A percent bar and a simple-interest explorer with sliders show how the numbers move.
How do you find a percent of a number? Change the percent to a decimal by dividing by 100, then multiply. For example, 35% of 480 is 0.35 × 480 = 168. On the calculator, 35 % × 480 does the same thing.
How do you calculate percent increase or decrease? Subtract to find the change, divide by the original (starting) amount, and multiply by 100. From $80 to $92 is a change of $12, and 12 ÷ 80 = 0.15, so a 15% increase.
Is 20% off and then 10% off the same as 30% off? No. The second discount comes off the lower sale price. You pay 0.80 × 0.90 = 0.72 of the price, which is 28% off, not 30%.
What is the simple interest formula on the GED? I = Prt, where I is interest, P is principal, r is the yearly rate and t is time. It is on the official formula sheet. Write the rate as a decimal and the time in years.
How do you use months in I = Prt? Divide the months by 12 to get years. For 18 months, t = 18 ÷ 12 = 1.5. Using 18 instead of 1.5 makes the interest 12 times too big.
Is percent on the GED formula sheet? No. Only simple interest is listed. The part, whole and percent relationships and percent change are skills to practise until they feel automatic.
Sources
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