To do this, change into common denominators. The common denominator is 21 so change the problem from 1/3 + 1/7 to 7/21 + 3/21. This gives you 10/21. It is already in simplest form.
you don't need to know this for real life situations but for school you do need to know it, i dont know how to do this but here's a link to the same question you have but it's explained on how to solve it
https://www.wyzant.com/resources/answers/499729/fireworks_can_go_up_to_1000_feet_in_around_3_seconds_what_s_is_it_s_average_vertical_speed_in_feet_per_second_and_miles_per_hour
((Also i'm a fellow ARMY as well =) ))
When you solve a proportion, your first step should be to cross-multiply.

=

When you cross-multiply, you should get this equation:
(2x)(3) = (x + 3)(5)
Now, solve it using algebra.
(2x)(3) = (x + 3)(5) Multiply
6x = 5x + 15 Subtract 5x from both sides
x = 15
The value of
x is 15.
Since the area of a circle is π

, and our radius is 9 we know the total area to be 254.469, and the ratio of sector area to total area is 169.56/254.469 = .6663. There are 360 degrees in a circle, so:

≈ 240°
240 degrees