275 million is what percent of 2.5 billion

[2.75 x 10⁸] / [2.5 x 10⁹ ]  = [2.75 / 2.5] x 10^-1  =  (11/10) x (1/10)  = 11/100 = 11%

Just divide 275,000,000 by 2,500,000,000, and multiply the decimal by 100 to get the percent.