Explanation :

 (A + B)'s 1 day's work =    1/12

 ; (B + C)'s 1 day's work =1/15

; (A + C)'s 1 day's work =    1/20

 Adding, we get: 2(A + B + C)’s 1 day’s work =1/12+1/15+1/20

=12/60=1/5

 (A + B + C)'s 1 day's work =1/10

 So, A, B and C together can complete the work in 10 days.

Explanation :

 Work done by A, B and C in 4 days ={1/10}x 4=2/5

. Remaining work =1 -2/5=3/5

 Now,3/5 work is done by B anc C in 10 days.             

 Whole work will be done by B and C in    10 x{5/3}=50/3 days.         

 (A + B + C)’s 1 day’s work =1/10

, (B + C)'s 1 day's work =    3/50

 A’s 1 day’s work =1/10-    3/50=2/50=1/25

  A alone could complete the work in 25 days.

Explanation :

(A + B)’s 1 day’s work =    1/45+    1/40

 =17/360

 Work done by B in 23 days ={1/40} x 23    =23/40

. Remaining work =1 -23/40

 =17/40

 Now,    17/360 Work was done by (A + B) in 1 day.         

17/40work was done by (A + B) in1 x{360/17}x17/40

  = 9 days.     

  A left after 9 days.

Relative angular displacements between two hands of clock in 1 min. is

5.5° / min.

∴ After 13 min. angle will be 210 – 5.5 × 13 = 138.5°

o r 360 – 138.5 = 71.5°