Question 1201799
Present value   $10000
Future value double = 20000
rate = 10%

Present Value=Future Value/(1+r)n

10000 = 20000/(1+0.1)^n

(1.1)^n = 20000/10000

1.1^n = 2

n = log2/log1.1

= 7.2725 years

How long will it take if he earns only 7% annual interest?

same formula same method 10.2448 years

it is clear you can deduce