If a company issues 2,500,000 shares with voting rights, how many shares must an investor buy to be assured control of the company?

If each investor receives voting rights for company decisions based on share ownership, every shareholder has 10% control. 
 
 If a company issues 2,500,000 = (approx)= 1,250,000 shares

example: If the company issues another 25,000,000 options or shares over the intervening five years so there are  50,000,000 shares at the IPO (typically either as part of fundraising including an IPO or to hire employees), you’re left with .01% – one basis point or half of your original percentage. You have had 50% dilution. You now make half as much for the same company value.

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a word that has the same meaning as another word is a synonym

Explanation:

for example a synonym of strong would be powerful since they nearly have the same meaning