Use Operands + or * for calculation

Dear Experts,

I have few mathematical operands stored in SAP custom fields. How do i use it for calculation?

Please take this as example.

ztest-field1 = '100'.

ztest-field2 = '*'.

ztest-field3 = N+n

Now my requirement is i need to pass numeric values to N & n. Since we have addition operand in between them values of N & n needs to be summed & then i need to have formula like below.

Ztotal = ztest-field1 ztest-field2 ztest-field3. (Here ztest-field2 will again be a mathematical operand * ). So that I will get total of field1 & field2.

Thanks & Regards,

Satty