1/2 km = 500m,  3/4 km = 750 m.

 

From question,

     500 = 50.u_a + 1/2. a. 2500

 or 500 = 50.u_a + 1250a ...................eq(1)

  u_b = u_a + 50a

Also,

  750 = 50u_b+ 1/2.a.2500

 750  = 50u_a + 2500a + 1250a ........................eq(2)

 

 (1) and (2) are the two equations in two unknowns u_a and a. Solve them to get the ans