The value of Vo depends on the source resistance of the voltage source Vi. If we call this resistance R, then in the absence of a load:
Vo = ((5 * R) + (Vi * 2490))/(2490 + R)
So it all depends on the value of R. If the voltage source is an Arduino pin followed by a simple RC network to smooth out the PWM, then R is the resistor used in the RC network plus about 25 ohms.