The 7th graders at Lincoln Avenue School were helping to renovate a playground for the kindergartners at a nearby elementary school. City regulations require that the sand underneath the swings be at least 15 inches deep. The sand under both swing sets was only 12 inches deep when they started. The rectangular area under the small swing set measures 9 feet by 12 feet and required 40 bags of sand to increase the depth by 3 inches. How many bags of sand will the students need to cover the rectangular area under the large swing set if it is 1.5 times as long and 1.5 times as wide as the area under the small swing set? Show all of your work. *

The students will need 90 bags of sand to cover the rectangular area under the large swing set.

Step-by-step explanation: First of all the area of the small and large swing sets is given, all measured in feet. The sand under both swing sets was supposed to be 15 inches deep, but was found to be only 12 inches deep. The rectangular area under the small swing set was measured as 9 feet by 12 feet which gives an area measurement of 108 feet squared. In order to fill up this area by an extra depth of 3 inches, the students had to use up 40 bags of sand.

They would need to also fill up the rectangular area under the large swing set by a depth of 3 inches. However, the dimensions are different because the area under the large swing set has its length 1.5 times as long as that of the small swing set and same thing applies to its width, which is also 1.5 times that of the small swing set. This means the dimensions of the large swing set are derived as follows;

Large swing length = 9 * 1.5

Large swing length = 13.5

Large swing width = 12 * 1.5

Large swing width = 18

If the length and width of the rectangular area under the large swing set are now given as L = 13.5 and W = 18, then the area becomes;

Area = L * W

Area = 13.5 * 18

Area = 243 feet squared

We can now apply the ratio of both areas of the different swing sets to determine the correct number of bags of sand needed for the area under the large swing set.

The ratio of both areas is derived as

Ratio = 108 : 243

Ratio = 4 : 9

Let x be the number of bags of sand needed for the area under the large swing set. You now have;

4 : 9 = 40 : x

This can be properly expressed mathematically as,

4/9 = 40/x

By cross multiplication you now have,

4x = 9 * 40

4x = 360

Divide both sides of the equation by 4

x = 90

Therefore, the students will need 90 bags of sand cover the rectangular area under the large swing set.

step-by-step explanation:

30% of $420 is 0.30($420) = $126 and $420 + $126 = $546.

or, 130% of $420 is 1.3($420) = $546.

notice that this is not the $600 original list price with which we started.

why doesn't this work?

the method does not work because 30% of $420 is not the same amount as the 30% of $600. it may seem obvious when stated this way, but it is a very common error made on problems of this type. we cannot just use the same percent and get back to the original amount because we are taking that percent of a different value.

to work problems of this type correctly, it is generally best to write an equation for the situation and then solve.

example: since (100% – 30%)(original price) = (sale price)

70% of the   (original price) is $420.

0.7x   = 420

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x = 600

the original cost was $600.

we may also solve this problem using the two-step method. working the problem in two steps requires us to use more algebra skills. we have to know that x = 1x (identity property of multiplicationtext annotation indicator) and that 1x – 0.3x = 0.7x (subtracting like terms). then we solve the problem as follows:

(original price) – 30%(original price) = sale price

x – 0.3x = 420

0.7x = $420

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x = 600

the original cost was $600.  

note that with both of the above methods we do obtain the correct original amount.

example: after a 5% pay raise, hermione is earning $22,680 per year. what was she earning before the pay raise?

105% of (original salary) = current salary

1.05x = 22,680

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x = 21,600

hermione earned $21,600 per year before the pay raise.

this is an example so maybe you will get how to at least do it!