Given:
Serenity picked 75% of the apples of a tree in her backyard.
She picked 30 apples.
To find:
The number of apples on the tree in the beginning.
Solution:
Let x be the number of apples on the tree in the beginning.
Serenity picked 75% of the apples of a tree in her backyard.
Number of apples picked = 75% of x
= 
= 
She picked 30 apples.

Multiply both sides by 4.


Divide both sides by 3.


Therefore, the number of apples on the tree in the beginning is 40.
Answer: Our required linear equation would be 
Step-by-step explanation:
Since we have given that
Number of cups = 150
Cost = $20
Number of cups = 400
Cost = $45
Let x be the number of cups, y be the cost .
We will use "Equation of line using Two point slope":

Hence, our required linear equation would be 
<span>b+c=<span>10/a</span></span><span>a(b+c)=10</span><span>a=<span>10<span>(/b+c)</span></span></span>