20 minutes =20÷60= 1/3 hours

Xiao Zhang's speed = Xiao Zhang's farther distance It takes time for Xiao Zhang to get to B earlier than Xiao Li.

=5÷( 1/3)

=15km/h

Xiao Li's speed = Xiao Zhang's speed-Xiao Zhang is faster than Xiao Li.

= 15-3

=12km/h

When Xiao Zhang arrived in B.

The distance from Xiao Li to B = Xiao Li's speed × the time when Xiao Zhang arrived at B earlier than Xiao Li.

= 12×( 1/3)

=4 km

The time for Xiao Zhang to arrive at B is 4 ÷ 3 = 4/3 hours.

AB distance = Xiao Zhang's speed × Xiao Zhang's time.

= 15×4/3

=20 km