Question 1031446
P=Po{1+r/t)^nt
P/Po=2 when it doubles.
2=(1+0.0425/4)^4n
ln of both sides 
ln2=4nln(1+(0.0425/4)); divide both sides by ln(1+(0.0425/4))
=65.58.  Divide that (still not rounded) by 4 to get n in years.
16.4 years (now round).
Notice that the 50,000 doesn't even need to appear.  Doubling the money is doubling the money.
Notice that the rule of 72 would say that doubling would take just under 18 years (for 4%) and 16.94 years for 4.25%.  The answer makes sense.
===========================
2=(1.05)^n
ln2= n ln (1.05)
n=14.2 years.