Dear student, 721 + 722 + 723 + 72425 = 721(1 + 7 + 72 + 73)25 = 721*40025∵400 is divisible by 25, so the remainder is zero for the whole expression Regards

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Dear student,

\frac{7^{21} + 7^{22} + 7^{23} + 7^{24}}{25} = \frac{7^{21} (1 + 7 + 7^{2} + 7^{3})}{25} = \frac{7^{21} * 400}{25} ∵ 400 i s d i v i s i b l e b y 25, s o t h e r e m a i n d e r i s z e r o f o r t h e w h o l e e x p r e s s i o n .

Regards

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