John was swimming for hour and a half hours at a speed of 2 miles per hour. How far did he swim?
2 answers:
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1 step: 
, 
, then
.
2 step:
, then
and
will have form 
.
3 step: Solve this system
and dividing first equation on second we obtain 
. So, 
and 
, 
- the common ratio.
4 step: Insertinto equation 
and obtain 
, from where 
.
2 step:
and
3 step: Solve this system
4 step: Insert