We know:
60 seconds #=# 1 minute. So multiplying something by:
#(1 min)/(60 sec)# is the same as multiplying by #1#
And, we know:
60 minutes #=# 1 hour. So multiplying something by:
#(1 hr)/(60 min)# is the same as multiplying by #1#
So we can rewrite 9,000 seconds as:
#9000 sec xx (1 min)/(60 sec) xx (1 hr)/(60 min)# is the same as multiplying 9000 seconds by #1# twice:
#9000 color(red)(cancel(color(black)(sec))) xx (1 color(blue)(cancel(color(black)(min))))/(60 color(red)(cancel(color(black)(sec)))) xx (1 hr)/(60 color(blue)(cancel(color(black)(min)))) =>#
#9000 xx 1/60 xx (1 hr)/60 => (9000 xx 1 xx 1 hr)/(60 xx 60) =>#
#(9000 hr)/3600 => 2.5 hr#
9,000 seconds is 2.5 hours.
