Original study example
A sales table has quantities 3 and 5, at unit prices $10 and $8. Plan a spreadsheet or program that computes each revenue and the total.
Work through the example
Multiply quantity by price in each row: $30 and $40. Sum the row revenues to get $70. Keep prices as input values so a change updates the total. Test a zero-quantity row and explain why summing unit prices alone would give the wrong result.