Iam a number between 7 000 000 and
8 000 000.
all my digits are odd.
all the digits in my thousands period are the same.
all the digits in my units period are the same.
the sum of all my digits is 31.
what am i?
give as many answers as you can.
what strategies did you use to find the mystery number?

First, start off by listing a few of the numbers that follow the first three clues to see if you can narrow it down.
7,111,111
7,333,333
7,555,555
etc.
Then, start adding up digits to see if you're getting close.
7+1+1+1+1+1+1 = 13
7+3+3+3+3+3+3 = 25
7+5+5+5+5+5+5 = 37
Since 7,555,555 is too high, we step it down to 7,555,333
7+5+5+5+3+3+3 = 31
7,555,333 will work as an answer, as well as 7,333,555, since it's the same amount when the digits are added together.

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction