Maths --- percentage

Let x be the total distance and v be the original speed.
Original time taken = x/v
If speed is increased by 25%, new speed = v(1+25%) = 1.25v
Time taken for the new speed = x/1.25v = 0.8x/v
Percentage change = (0.8x/v - x/v) / (x/v) * 100%= -0.2*100% = -20%
It saves 20% of time taken

Let v, s, t, t1 be the speed, distance, initial time taken, final time taken respectively

v = s / t

When speed is increased by 25%,

(1 + 25%)v = s / t1
125%(s / t) = s / t1
t1 = t / 1.25
t1 = (4 / 5)t = 80%t

Thus when speed is increased by 25%,
time taken is decreased by 20%.