She gets a base pay of $80, so you must have a plain 80 in the expression.
80
Then she gets 4% of the value of merchandise she sells above $400. If she sells t dollars of merchandise, the amount that exceeds $400 is t - 400. For example, if she sold $500, then the amount that exceeds $400 is $100, and t - 400 = 500 - 400 = 100, so you see the expression t - 400 does give the amount of sales exceeding $400. She gets 4% of the amount that exceeds $400, so she gets 4%(t - 400) = 0.04(t - 400). Now we add this to the fixed 80 we had from above.
The correct expression is:
80 + 0.04(t - 400)
Joan's error is that she switched the 80 and the 400 in her formula.
Now we calculate her daily salary for sales of $575 in one day.
80 + 0.04(t - 400) =
= 80 + 0.04(575 - 400)
= 80 + 0.04(175)
= 80 + 7
= 87
She will make $87 for that day.
