Three times a certain number plus 6 is equal to twice the number plus 15. What is the number?

Answer 1

3*y =3y
3y+6=21
3y=21-6=10
3y=10

Answer 2

x = number

Three times a certain number plus 6 is equal to twice the number plus 15, means:

3 x + 6 = 2 x + 15

Subtract 2x to both sides.

x + 6 = 15

Subtract 6 to both sides.

x = 9

The number is 9.
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Proof:

3 x + 6 = 2 x + 15

3 • 9 + 6 = 2 • 9 + 15

27 + 6 = 18 + 15

33 = 33
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