A single discount equivalent to the series of discounts 15%, 10% and 5% is equal to:

27.325%

Three successive discounts of a%, b%, c% = (Original price) x $\frac{100-a}{100} \times \frac{100-b}{100} \times \frac{100-c}{100}$

When a = 15%, b = 10%, c = 5%

The single equivalent discount = $\frac{100-15}{100} \times \frac{100-10}{100} \times \frac{100-5}{100}$

                                            = $\frac{85}{100}  \times \frac{90}{100} \times \frac{95}{100}$   = $\frac{726750}{1000000}$

                                            = $\frac{100-27.325}{100}$ or 27.325%