Continuous compound interest is given by Aeʳᵗ where A is the amount, r the annual rate and t the time in years. So A=300000 and r=0.15.
(a) t=35: 300000e⁵˙²⁵=$57,169,880.54
(b) t=47: 300000e⁷˙⁰⁵=$345,857,622.80
(c) An increase of 1.75 is a growth of 2.75, which is 825000/300000.
e⁰˙¹⁵ᵗ=2.75, t=ln(2.75)/0.15=6.744 years approx.