**Thank you so much, Alex. Although I do not understand these packages, it worked :) thank you again. But I had another issue with Currency. Can not I use more than one Context ? since after adding **

Context("Numeric")->variables->add(y=>'Real');

parser::Assignment->Allow;

**I go an error from the " Currency" context. it is in bold . **

DOCUMENT(); # This should be the first executable line in the problem.

loadMacros(

"PGstandard.pl", # Standard macros for PG language

"MathObjects.pl",

"PGML.pl",

"contextCurrency.pl",

"PGcourse.pl",

"parserImplicitEquation.pl",

"parserAssignment.pl",

);

TEXT(beginproblem());

**Context("Currency");**

$showPartialCorrectAnswers = 1;

Context("Numeric")->variables->add(y=>'Real');

parser::Assignment->Allow;

$n=random(1,10,1);

$b=random(1,20,1);

$c=random(50,500,1);

$a=$n*$b;

$k=$a/$b;

**$d=Currency($k*$c);**

$f = Formula("y=$k*x");

BEGIN_PGML

State sales tax [`y`] is directly proportional to retail price [`x`]. An item that sells for [$a] dollars has a sales tax of [$b] dollars. Find a mathematical model that gives the amount of sales tax [`y`] in terms of the retail price [`x`]

The equation is [_______________]{"$f"}

What is the sales tax on a [$c] dollars purchase?

The answer is [_______________]{$d}

END_PGML

ENDDOCUMENT(); # This should be the last executable line in the problem.