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To reckon means to count. Its use as a synonym for "to think" in general is non-standard.
count, total, think, guess, judge, gauge
No. It's Latin, computare - to count, reckon, sum up.
count up, total, reckon, compute, add up, include, attach, supplement, adjoin, affix, append
add up, total, reckon, calculate, compute, tally, number
consider, judge, regard, deem, rate