: "
This is my homework and i couldn't do  it,please help me.
I wait your helpsss
On the planet Zephod, which orbits the star Betelgeuse, the sharks
increase at a rate of 5% of the guppy population per day, provided
there are 50 or more guppies per shark. Otherwise, the sharks die off
at a rate of 50% per day. The guppies increase at a rate of 80% per
day, provided the shark population is less than 20 % of the guppy
population. Otherwise there is no increase in the guppy population.
Each shark eats 5 guppies a day. Write a C program that allows the user
to input the initial shark population, the initial guppy population,
and the number of days of observation is to cover. The program is to
output a day-by-day account of the populations until one of them dies
off or the end of the observation period is reached. All reproductions
and deaths occur overnight. Any fractional fish are discarded. (These
sharks only feed during the day.)

Sample run:

Please enter number of sharks: 54
Please enter number of guppies: 1000
Number of days to observe: 15

Start of day 1          sharks 54         guppies 1000
End of day   1          sharks 54         guppies 730
Start of day 2          sharks 27         guppies 1314
        .
        .
        .
"

_commas () 
{ 
    _COMMAS=;
    _TH_SEP=,;
    c_num=$1

    while :
    do
        case $c_num in 
            ??? | ?? | ?)
                _COMMAS=${c_num}${_COMMAS:+"$_TH_SEP"$_COMMAS};
                break
                ;;
            *) left=${c_num%???};
               _COMMAS=${c_num#${left}}${_COMMAS:+"$_TH_SEP"$_COMMAS};
               c_num=$left
               ;;
        esac;
    done;
    _COMMAS=${_COMMAS}
}


## On the planet Zephod, which orbits the star Betelgeuse,
## the sharks increase at a rate of 5% of the guppy population per day,
## provided there are 50 or more guppies per shark.
## Otherwise, the sharks die off at a rate of 50% per day.
shark_change()
{
    if [ $(( $guppies / $sharks )) -ge 50 ]
    then
      shark_increase=$(( $guppies / 20 ))
      sharks=$(( $sharks + $shark_increase ))
    else
      sharks=$(( $sharks / 2 ))
    fi
    [ $sharks -le 0 ] && sharks=0
}

## The guppies increase at a rate of 80% per day,
## provided the shark population is less than 20 % of the guppy population.
## Otherwise there is no increase in the guppy population.
guppy_increase()
{
    if [ $(( $guppies / $sharks )) -ge 5 ]
    then
      guppy_plus=$(( ($guppies * 80 ) / 100 ))
      guppies=$(( $guppies + $guppy_plus ))
    fi
    [ $guppies -le 0 ] && guppies=0
}

## Each shark eats 5 guppies a day.
guppy_gobble()
{
    guppies_eaten=$(( $sharks * 5 ))
    guppies=$(( $guppies - $guppies_eaten ))
    [ $guppies -le 0 ] && guppies=0
}

## Write a C program that allows the user to input
##   the initial shark population,
##   the initial guppy population,
##   and the number of days of observation is to cover.
init()
{
    printf "Please enter number of sharks: "
    read sharks

    printf "Please enter number of guppies: "
    read guppies

    printf "Number of days to observe: "
    read duration
}

print_it()
{
  [ -z "$silent" ] && {
   _commas "$day"; day_c=$_COMMAS
   _commas "$sharks"; sharks_c=$_COMMAS
   _commas "$guppies"; guppies_c=$_COMMAS
   printf "%10s of day %${#duration}s %23s sharks  %26s guppies\n" \
                      "$time" "$day_c" $sharks_c $guppies_c
  }
}

exit_test()
{
  [ $sharks -le 0 ] ||
  [ $guppies -le 0 ] && exit
}

case $1 in
    -b) sharks=${2:-54}
        guppies=${3:-1000}
        duration=${4:-15}
        ;;
    * ) init ;;
esac

# The program is to output a day-by-day account of the populations
# until one of them dies off or the end of the observation period is
# reached.
# All reproductions and deaths occur overnight.
# Any fractional fish are discarded.
# These sharks only feed during the day.
day=0
while [ $day -lt $duration ]
do
  day=$(( $day + 1 ))
  time=Start
  print_it
  exit_test
  guppy_gobble
  time=End
  print_it
  exit_test
  guppy_increase
  shark_change
done