word problem

[dolor]

Lisa is training for a long-distance cross training skiing event. On October 1, she skied a distance of 2 miles. On each subsequent day, she skied 1.5 miles farther than she skied on the previous day. On what day did she ski more than 30 miles?

[lschaeff1]

O nthe 20th she skied 30.5 miles, unless she had to ski back, which means that on the 10th she skied 15.5 miles each way...

[sql_lall]

here's the logic:

Day 1: 2 miles
Day 2: 3.5 miles
...
Day n: 2 + (n-1) * 1.5 miles

which simplifies to 0.5 + 1.5*n miles

=> 0.5 + 1.5*n > 30 (thats what ur looking for)
=> 1.5*n > 29.5 (simplification)
=> n > 29.5 / 1.5 (getting unknown by itself)
=> n > 19.6666

so, the first day which she goes more than 30 miles is day 20.
Day 1 = Oct. 1st
so Day 20 = Oct. 20th

Oh, and also the question doesn't ask the FIRST day, only 'on what day', so technically any answer after October 19th is right, though safest to put the 20th

[dolor]

i forgot the (x-1)...makes a big diff