You likely cannot hold them to 21 days if you have in fact only earned 21 hours. Typing/writing "days" instead of "hours" is a typo, and it is easy to verify the correct amount--and you evidently know the correct amount, and so cannot even claim that you in good faith thought it was 21 days. The law will not enforce obvoius errors, but rather will correct contracts to what the facts show was intended.