.
These values of t are the roots of the equation
H(t) = 20, which is -5t^2 + 50t = 20.
Write it in the standard form
5t^2 - 50t + 20 = 0.
Solve it using the quadratic formula.
The two roots will be your answers.
The problem has two answers: one for for the model moving up and another for the model falling down.
Having my instructions, you do the rest.
If you still have questions or problems with it, then let me know.
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Comment from student: Thank u. I got 0.42 and 9.58. Is that correct ?
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My response : YES (!) It is CORRECT (!)
Nice job !
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As my reward and a bonus to you for your nice job, here is the list of my lessons in this site related to similar problems
- Problem on a projectile moving vertically up and down
- Problem on an arrow shot vertically upward
- Problem on a ball thrown vertically up from the top of a tower
- Problem on a toy rocket launched vertically up from a tall platform
Consider these lessons as your textbook, handbook, tutorials and (free of charge) home teacher.
Read them attentively and learn how to solve this type of problems once and for all.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this textbook under the topic "Projectiles launched/thrown and moving vertically up and dawn".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.