This is a "multiply it out" problem at least for the first few.
I hope you could do the multiplications without a calculator!
The first three are:
- 7 x 9 = 63
- 77 x 99 = 7,623
- 777 x 999 = 776223
Each time there's another 7
and another 2. The number of 7's and 2's is one less than the number of
digits in
each of the numbers we multiplied.
So we expect 77,777 x 99,999 = 7,777,622,223, and we can verify that
this is correct by multiplying it out.
In
order to get 77,762,223, we must multiply a number with four 7's by one
with four 9's, so it is 7,777 x 9,999.
Find these products: 7 x 9, 77 x 99, 777 x 999.