Maths problem, how to do, thanks?

Let Jack's original speed = s, original time required = t
s=30/t - - - - -→ 《s=d/t》
t=30/s. . . . . . . . . . . . . ①

new speed = s+3, new time required = t - ½
s+3 = 30/(t - ½)
t - ½ = 30/(s+3). . . . . . . . . . . . . ②

①-② :
½ = 30/s - 30/(s+3)
1 = 60[1/s - 1/(s+3)]
1/60 = [(s+3) - s] / s(s+3)
1/60 = 3 / s(s+3)
s(s+3) = 180
s²+3s-180 = 0
(s-12) (s+15) = 0
s=12 or s= -15(rejected)
∴ original speed, s = 12 km/h