How do I write the equation to get the answer ?

 Take the information given in the situation. Consider the relationships between the amounts, numbers and unknowns that are in the description.

If the word "is" or "are" is included in the description, that is equivalent to an equal sign.

Words like "and," "also" "more than" and "combined" are clues that there may be addition. "Total" often means the sum of an addition equation.  

"From" and "difference" are clues that there is subtraction.

"Times" "by" and "of" art clues for multiplication. "Find the Area of. . ." dedinitely requires multiplication.

"Ratio," 'into' "per" and "among" are clues to division.

Use a letter x, y, d, t, or whatever helps identify the

Step-by-step explanation: Without a specific situation, it is impossible to give a specific answer.

Examples:  The total number of seats in the auditorium is 125. 75 tickets have been sold already. How many more tickets are left. (This is the unknown)

The Total is  the given 75 plus the unknown.

125 = 75 + x   is the equation to start with

What is three-fourths of twelve?  "What" is the unknown.

x = 3/4 × 12  is the  equation to start with.

The ratio of red marbles to blue marbles is 2:3  There are 25 marbles all together. How many red marbles and how many blue marbles.

In this question there are two equations

2r + 3b = 25 This is the equation for the total    r = 2b/3 is the division

Substitute the value of "r" from the second equation to solve the first equation

2(2b/3) + 3b = 25

c. (3,2,6) is the answer.