The time interval in which your friend beats you is determined as 4.2 minutes.
<h3>Time in which you catch up with your friend</h3>
The time interval in which your friend beats you is determined from the principle of relative velocity.
(Va - Vb)t + (Vc - Vd) = d
(6 - 3)t - (6 - 9)t = 1500
6t = 1500
t = 1500/6
t = 250 s
Time in minutes = 250 s/ (60s/min) = 4.2 minutes
Thus, the time interval in which your friend beats you is determined as 4.2 minutes.
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if rebecca stands in front of david at a distance of do and david perceives the position of rebecca at di, di will be +84 cm
<h3>What is focal length ?</h3>
How strongly light converges or diverges depends on an optical system's focal length, which is the inverse of optical power. A system with a positive focus length is said to converge light, whereas one with a negative focal length is said to diverge light.
focal length = +60 cm
magnification m = -0.40
focal length being positive an magnification negative.
given lens is a convex lens.
for a lens
m = di/do and 1/f = (1/di) - (1/do)di
= -0.4do1/f = (1/-0.4do) - 1/do do
= -210 cmdi = -0.4 * -210
di = +84 cm
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The answer is B, because coal is a non renewable recource