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.
