Question 969299
P=Po {1 + (r/n)}^nt  but n=1 compounding, every year, and t=20 years

P=10,000{1+ (.05)}^20=$26,532.98

P=5000*exp^(.10*20)=$36,945.28  P=Po (starting)* exp (r*t).  This is  5,000 * (exp(2)

If you want to estimate, figure that in the first instance, you get about $500 interest a year for 20 years. That is $10,000, an underestimation to be sure, but for the first several years, you won't see much of a change from $500.

In the second, you can invoke the rule of 72, where 72/interest rate in per cent is time in years it takes to double money.  Here, money doubles in 7.2 years, and it doubles again and almost a third time.  You are very close to $40,000 with the third doubling (21.6 years), so you can estimate reasonably well.