Explanation:

Efficiency of A is 1.5 time more than B,

so, efficiency ratio of A: B = (1+1.5) : 1 = 2.5 : 1 = 5:2

efficiency ratio of A: C = 1 : 2= 5:10

so efficiency ratio of A: B: C= 5:2:10

work completed by A & B in 7(1/2) days= (5+2) x 7(1/2)= 52(1/2) unit

time taken by B & C to complete this work = 52(1/2)/(2+10)=35/8=4(3/8) days