Question

5 cows or 15 horses can graze a grass field in 8 days . In how many days 8 cows and 11 horses will graze twice of this grass field ?

Answer 1

5 cow =15 horses
1 cow =3 horses

now ,8 cow =3×8=24 horse
8 cow + 11 horse =24 +11
 ⇒8 cow + 11 horse=35 horse

15 horse graze the fied in  8 day
1 horses graze  the field in (15×8) day

35 horse  graze the same field in = 15×8/35=24/7    day

         If field is twice then days also goes twice
 so , therefore number of day=24×2/7= 48/7 days

Answer =48/7 days 

Answer 2

5 cows graze in 8 days
1 cow graze in 40 days
8 cows graze in 5 days ...( 1 )

15 horses graze in 8 days
1 horse graze in 120 days
11 horses graze in 120/11 days ...( 2 )

Adding ( 1 ) and ( 2 )

We get

5 cows and 11 horses graze 2 times of field in 5 + 120/11 days
( 55 + 120 )/ 11 days
175/11 days