The way to share variables between the main report and the subreport is to create a formula in both the main report and subreport and declare the same shared variable in both formulas. Then they will share their data.

For example,
Shared NumberVar TxTotal;

Then in the formulas summarize the data appropriately.

This is what I did:

Create two separate reports WagesReport.rpt which sums up all wages transations for each employee and AllowancesReport.rpt which sums up all allowance transactions for each employee.

I include AllowancesReport as a subreport of WagesReport and place the allowances total in the details section along with the wages total name etc. fields.

My report now looks like the following, which is fine:

      Name
First         Last  SSN    Bank  Account#  Wages  SS     Allowances
John        Jones1234  Scotia 22-333     230.00  9.00  400.00   

I proceeded to add the column GrossPay which is the sum of Wages + Allowances.  While editing WagesReport I right click on the Allowance subreport and   select the edit subreport option.  I then create a new formula field called frmAllowances and in the formula editor I type the following:

Shared NumberVar shrAllowances;
shrAllowances := {ALLOWANCES.ALLOWANCES};

{ALLOWANCES.ALLOWANCES} is the field that represents the total allowances.

I save the subreport.

I then edit WagesReport by creating a new formula field called GrossPay which is supposed to sum up the wage amount and allowance amount.  In the formula editor I type the following formula for GrossPay:

Shared NumberVar shrAllowances;
{WAGES.WAGES} + shrAllowances;

I insert GrossPay in the details section of my report and expect to get the following:

Name
First         Last  SSN    Bank  Account#  Wages  SS     Allowances Gross Pay
John        Jones1234  Scotia 22-333     230.00  9.00  400.00        630.00

Instead I get this:

Name
First         Last  SSN    Bank  Account#  Wages  SS     Allowances Gross Pay
John        Jones1234  Scotia 22-333     230.00  9.00  400.00        230.00

Where have I gone wrong?