`5/7` of a number is 15. What is the number?

8 Answers | Add Yours

baxthum8's profile pic

baxthum8 | High School Teacher | (Level 3) Associate Educator

Posted on

Define the number by the variable x.

Therefore 5/7 of "what number" is 15 means `5/7* x = 15`  

In order to isolate the variable x, we can multiply both sides by the reciprocal of `5/7`  which is `7/5.`

A number multiplied by its reciprocal is always equal to 1.

`7/5* 5/7 x = 15* 7/5`

`x = 21`

`5/7` of 21 is 15.

justaguide's profile pic

justaguide | College Teacher | (Level 2) Distinguished Educator

Posted on

Let the number 5/7 of which is 15 be X.

(5/7)*X = 15

=> X = 15*(7/5)

=> X = 21

5/7 of 21 is equal to 15

sid-sarfraz's profile pic

sid-sarfraz | Student, Graduate | (Level 2) Salutatorian

Posted on

Question:-

5/7 of a number is 15. What is the number?

Solution:-

Let x be the unknown number;

5/7 of x = 15

`5/7*x=15`

Multiply 7 to both sides;

`5/7(x)*7=15*7`

`5x=105`

Divide 5 from both sides;

`5x/5=105/5`

`x=105/5`

`x=21`

Hence the unknown number is 21.

We can recheck by inserting the x value in;

5/7 of x = 15

`5/7*21=15`

`5*3=15`

`15=15`

`LHS=RHS`

Hence proved!

Therefore 5/7 of 21 is 15.

malkaam's profile pic

malkaam | Student, Undergraduate | (Level 1) Valedictorian

Posted on

5/7 of a number is 15.What is the number?

Let that number be x, i.e the unknown variable,

5/7 * x = 15

5/7x * 7/5 = 15 * 7/5        Multiplying both sides by the reciprocal of 5/7

x = 21 Answer.

To check the answer insert the value of x in the equation,

5/7 * 21 = 15

15 = 15

LHS = RHS

Proved. 

jess1999's profile pic

jess1999 | Student, Grade 9 | (Level 1) Valedictorian

Posted on

To find 5/7 of a number is 15 first write the equation

5/7x = 15 you have to try to get " x " alone. In order to get " x " alone, you have to divide by 5/7 on both sides.

By dividing your equation should look like

x = 105/5 simplify

x = 21

acompanioninthetardis's profile pic

acompanioninthetardis | Student, Undergraduate | (Level 1) Valedictorian

Posted on

5/7 X(unkown number)= 15

multiply by the recipricol of 5/7, which is 7/5 on both sides to get the value of x so we have

X= 15/1* 7/5

15*7=105

___________

5*1=5

so you have 105/5=X

simplify by dividing to get 21=X

nisarg's profile pic

nisarg | Student, Grade 11 | (Level 1) Valedictorian

Posted on

 of a number is 15. What is the number?

15/5=3

one unit is 3 then 7 units is 21 so the number is 21

We’ve answered 318,915 questions. We can answer yours, too.

Ask a question