mind_trainer.sh

#!/bin/bash - 
#===============================================================================
#          FILE: Mind Trainder
#         USAGE: ./mind_trainer.sh 
# 
#   DESCRIPTION: Train your mind to quickly find your income as a freelancer.
#        AUTHOR: Dr. Sandeep Sadanandan (sillyfellow), grep@whybenormal.org
#===============================================================================

set -o nounset                              # Treat unset variables as an error

function my_random() 
{ 
  echo $(shuf -i $1-$2 -n 1)
}

# Of course, you could have rates like 55.5 per hour. 
# But I am going with multiples of five. 
function five_floor()
{
  NUMBER=$1
  ffREM=$((NUMBER % 5))
  echo $((NUMBER - ffREM))
}

function check_correctness()
{
  EXPECTED=$1
  begin_time=`date +%s`
  read COMPUTED < /dev/tty 
  end_time=`date +%s`
  TIMETAKEN=$((end_time - begin_time))
  CORRECTNESS="Wrong!! (expected: $EXPECTED)"
  [ $EXPECTED  -eq $COMPUTED ] && CORRECTNESS="Correct!!!"
  echo "You took $TIMETAKEN, and you were completely $CORRECTNESS"
}

function evaluate_hours() 
{
  DAYS=$1
  echo "Find the total hours of work in $DAYS days"
  check_correctness $((DAYS * 8))
}

function evaluate_netto()
{ 
  DAYS=$1
  RATE=$2
  echo "Now tell me your netto at a rate of $RATE!"
  check_correctness $((DAYS * 8 * RATE))
}

function evaluation_round() 
{ 
  DAYCOUNT=$(my_random 8 18)
  HOURRATE=$(five_floor $(my_random 70 131))
  evaluate_hours $DAYCOUNT 
  evaluate_netto $DAYCOUNT $HOURRATE 
}

while true 
do 
  evaluation_round 
  echo ""
done 

Example output
Once wrong, and once correct.

~/g/s/g/M/energis_app_server » ./mind_trainer.sh
Find the total hours of work in 16 days
0
You took 3, and you were completely Wrong!! (expected: 128)
Now tell me your netto at a rate of 110!
0
You took 3, and you were completely Wrong!! (expected: 14080)

Find the total hours of work in 16 days
128
You took 13, and you were completely Correct!!!
Now tell me your netto at a rate of 125!
16000
You took 8, and you were completely Correct!!!

Find the total hours of work in 16 days
^C
~/g/s/g/M/energis_app_server »

Of course, no real error checking.