How do I write a formula for the following salary bands please

For salaries between zero and 30,000 = salary x 95%

For salaries between 30,001 - 70,000 = 30,000 x 95% + balance x 92.5%

For salaries over 70,000 = 30,000 x 95% + 40,000 x 92.5% + balance x 90%