A store advertises that customers can take 25%off the original price and then take an extra 10%off is this 35% of? explain

No, it is not 35% off ... Let me explain. Let's say the original price is $100 to make the math easy. When you take 25% off then take an additional 10% off, the 10% is off of the discounted price, so

100 x 25% = 100 x. 25 = 25 dollar discount

New price = $75

Now 10% off of $75

10% x 75 =.10 x 75 = $7.50 discount

New price = 75 - 7.50 = $67.50

Now, if you just took 35% off of $100

100 x 35% = 100 x. 35 = $35 discount

New Price = 100 - 35 = $65

Bottom line ... when you take an "extra" percentage off, you are taking a percentage of a smaller number, so the discount is not as big.