Question 378491
Use the formula {{{B = P(1+r/n)^(nt)}}}.  Now n = 1, r = 0.045, P = 7000.  we want

{{{28000 = 7000(1+0.045)^t}}}, or {{{4 = 1.045^t}}}.

Then ln4 = t*ln1.045, or t = ln4/ln1.045 = 31.49 years.