Or:
=SUM(INDIRECT("Sheet1!$C" & (ROW(L26)-26)*7+1 &" :$BB"& (ROW(L26)-26)*7+7))
Or:
=SUM(INDIRECT("Sheet1!$C" & (ROW(L26)-ROW(L$26))*7+1 &" :$BB"& (ROW(L26)-ROW(L$26))*7+7))
Depending on whether or not you want to see the numbers change.
Regards
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