The answer is 3.85555556 because the ratio is 9:1 the length 34.7 so that means the length will be 9 times the width so you divide.
15 girls 12 boys 5x3=15 and 4x3=12 so 15 girls 12 boys

is your formula
0.17=1.7e^{0.1t} 10% of 1.7 is 1.7*0.1=0.17
0.1=e^{0.1t}
ln(0.1)=0.1t
t= ~23 years
if I assume the values used are correct, it will take approximately 23 years for 10% to decompose.