An innovative rural public health program is reducing infant mortality in a certain West African country. Pretend the program in Senegal has been reducing infant mortality at a rate 7.3 % per year. How long will it take for infant mortality to be reduced by 39 %?

100 - 7.3 = 92.7 %

so
next year mortality rate = 0.927 * this year

100 - 39 = 61
so we want to reach 0.61

0.927^n = 0.61
n log 0.927 = log 0.61
n = -0.21467 / -0.03292
n = 6.5 years