Hello from MrBillDoesMath!
Answer:
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
