Question

Please let me know the correct answer.

Answer 1

Set h to 640 and solve for t:

640 = -490t^2 + 1120t

Subtract 640 from both sides:

-490t^2 + 1120t - 640 = 0

The formula to solve a quadratic equation is:

x = -b -/+ sqrtroot (b^2-4ac)/(2a) where a = -490, b = 1120 and c = -640

Solve:

x = -1120 -/+ sqrtroot (1120^2-4(-490)(-640) )/ 2(-490)

x = 8/7 = 1.1428 = 1.14

Time was 1.14 seconds