Calculating Weighted Average: Using variables

Rachel's grades are are follows:

95 on an exam that represents 20%.
89 on an exam that represents 15%.
78 on an exam that represents X%.
92 on an exam representing the remaining percentage.

What is Raachel's average grade?

$92.15-0.14x$

A truck travels for 4 hours at a speed of 30 km/h, then for 3 hours at a speed of 50 km/h.

If its average speed during 15 hours is 5X km/h, then what is its speed after the first 7 hours of travel?

$9.375x-33.75$ km/h

In a city, they decide to build some new parks.

47 plants were planted in 4 parks.

38 plants were planted in 9 parks.

Parks: y
Plants: x

How many plants were planted on average in each park?

$\frac{530+xy}{13+y}$

A biology class receives the following grades:

30% of the students obtained 75.

20% of the students obtained 68.

X of the students obtained 94.

The rest obtained 53.

What is the class average?

$62.6+0.41x$

A mixture contains 3 gases:

Helium constitutes 3% of the mixture and costs $7 per 100 grams.<br><br>Hydrogen constitutes 87$11 per 100 grams.

If the mixture sells for $X per 100 grams, then what is the price of hydrogen?

$1.15x-1.51$ $

Here are Armando's grades in English literature:

X in an exam that represents 40% of his final grade.

Y in an exam that represents 0.25Y% of his final grade.

83 in an exam that represents 20% of his final grade.

78 in an exam that represents the rest of his final grade.

What is Armando's average grade in English literature?

$0.4x+\frac{y^2}{400}+47.8-0.195y$

In a apartment block there are 20 apartments.

5 apartments house 4 tenants each.

6 apartments house 3 tenants each.

The rest of the apartments house 5 or 7 tenants.

On average each apartment houses$y-2$ tenants.

How many apartments are there where 5 tenants live?

$70.5-10y$

On a shelf there are 17 books with 450 pages, 10 books with 344 pages, and $8x+3$ books with 417 pages.

On average, each book on the shelf has $206.663x$ pages.

Calculate X.

$2$