We have the triangle 30° - 60° - 90°.
The lenght of the sides are in proportion: 

Answer:
12 days
Step-by-step explanation:
420/35 to find amount of days to finish book
420/35 = 12
I'm not sure what your variables are, but I will try task 3 with you.
y=$500(in billions)
x= the number of years that have passed since 2009
percentage of decrease: 1 - % =
so your formula will look like this: y(1 - %)^x