The formula : Pn+1 = a * Tn + (1 - a) * Pn ...... (i)
Given, exponential average of the measured lengths of previous CPU bursts is, a =0.25 .
a = 0.25, T1 = 4, T2 = 12, T3 = 8 and P1 = 4 all these values are provided, now put them into the formula (i)
So, we have
P2 = 0.25 * 4 + 0.75 * 4 = 4
P3 = 0.25 * 12 + 0.75 * 4 = 6
P4 = 0.25 * 8 + 0.75 * 6 = 6.5
Answer is 6.5 units.