Estimate the life expectancy of a 5-solar m, 20-solar luminosity star.?

If a star has 20 times the luminosity of the Sun it means that it is consuming its hydrogen 20 times faster than the Sun. If it has only 5 times the m, then it will "run out of fuel" 4 times faster than the Sun (20/5). If Sun's life expectancy is 10 billion years, then the bigger star will last approx. 10/4 = 2.5 billion years.