Here's the formula for an exponential RC discharge.
(If you were not a Brainly robot, you would have seen this formula before,
in any class that would assign you this problem to solve.)
Charge = (initial charge) · e ^ -(time / RC) .
In this situation:
0.1 = (1) · e ^ -(.0025 / 2.8R x 10⁻⁶)
Ln of each side: ln(0.1) = -.0025 / 2.8R x 10⁻⁶
Multiply each side
by R R · ln(0.1) = -.0025 / 2.8 x 10⁻⁶
Divide each side
by ln(0.1) = -2.303: R = 0.0025 / (2.303 · 2.8 x 10⁻⁶)
I get 387.8 Ω
