Hey!
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![\boxed{ \bf 100~+~25x~=~m}](https://img.qammunity.org/2020/formulas/mathematics/middle-school/ta50uzm5bds2cb610h6ph9ob8ebh4egdnk.png)
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Solution:
m = money at the end of summer.
x = how many days are in summer.
If he starts with 100 dollars at the beginning of summer, then you just want to add it depending on how many days he gets paid $25.
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Hope This Helped! Good Luck!