Then the distance from the person in the first row to the bottom of the flagpole is s 1=h/tan60.

The distance from the person in the last row to the flagpole is s2=(h- 10)/tan 30.

Square root of S2-s1=106cos15.

The square root of (h- 10 radical 6sin15)/tan30-h/tan60 =106cos15.

Root number 3h- 10 6 * root number 3 (root number 2 of 6) /4-h* root number 3/3 = 10 6 * (root number 2 of 6)/4.

(2/3 root number 3)h- 15 root number 3+ 15= 15+5 root number 3

H=(20 elements 3)*3/(2 elements 3) = 30m.

Therefore, the average speed v = h/t = 30/50 = 3/5m/s.

Option a