Ithink of a number double it add 15 my number is 21 what number was i thinking about

Hello from MrBillDoesMath!

3

Discussion:

Let the number be "n",

2n + 15  = 21          =>  subtract 15 from both sides

2n = 21 - 15 = 6     =>   divide both sides by 2

n = 6/2 = 3

Check: Start with 3. Double it (gives 6), add 15 to it ( 6 + 15)  which gives 21/

Thank you,

MrB

2

step-by-step explanation: