Solution
$$\textbf{Step-1: Write the least & greatest number of seven digits and simplify}$$ $$\Rightarrow$$ $$\text{The smallest}$$ $$7-$$ $$\text{digit number}$$ $$= 1000000.$$ $$\Rightarrow$$ $$\text{The greatest}$$ $$7-$$ $$\text{digit number}$$ $$= 9999999.$$ $$\Rightarrow$$ $$\text{Total}$$ $$4-$$ $$\text{digit numbers}$$ $$=(9999999 - 1000000)+1$$ $$\Rightarrow$$ $$89,99,999+1$$ $$=90,00,000$$ $$\text{So, we say that we have}$$ $$90,00,000$$ $$\text{seven-digit numbers.}$$ $$\textbf{Hence, answer is 90,00,000}$$
