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It is divisible by 25 so there is no remainder
normal way:
 \frac{7^{21}+7^{22}+7^{23}+7^{24}}{25}=  \frac{223418345633313602800}{25}= 8936733825332544112
smart way:
 \frac{7^{21}+7^{22}+7^{23}+7^{24}}{25}=  \frac{7^{21}(1+7+49+343)}{25}= \frac{7^{21}(400)}{25}=4(7^{21})
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