A stone is thrown upwards. It's height h in metres after t seconds is given by the formula h=30t-5t². Find t when it is at a height of 25m.
To find t when the height is 25m, we can set the equation h = 30t - 5t² equal to 25 and solve for t.
30t - 5t² = 25
Rearranging the equation, we get:
5t² - 30t + 25 = 0
Now, we can solve this quadratic equation for t by factoring or using the quadratic formula. Factoring the equation, we get:
5t² - 25t - 5t + 25 = 0
5t(t - 5) - 5(t - 5) = 0
(5t - 5)(t - 5) = 0
t = 1 or t = 5
Therefore, the stone will be at a height of 25m at t = 1 second and t = 5 seconds.