If someone has a pay stub with the total hours of $21\mathrm{.}50$ with $7\mathrm{.}25$ hours being the differential pay of $$2\mathrm{.}22$ and $14\mathrm{.}25$ hours with the base pay of $$22\mathrm{.}15$ What would be my gross pay of that week?

Can you elaborate more on why the differential rate is multiplied by the hours rather than adding the differential rate by the base pay then multiplying it by the differential hours then adding the calculated differential plus the base pay?