Fractions help needed grade 8 level

Hi there, i need the grade 8 level fractions help. homework is below. Thank you!!

twice a week i go swimming and swim the same number of lengths of the pool each time.

as i swim, i count the lengths i've done so far, and make it into a fraction of the wohle number of lengths i know i'm going to swim.

For example, if i swam 10 lengths each time, as i finished the 5th length i'd think that's 5/10.

If i make that into a simpler fraction, I get 1/2. after 8 lengths it would be 8/10 or 4/5.

But I don't swim 10 lengths. how many do i swim??

here's the clue you need to work it out...

After the first length, the only numbers of lengths that can't be changed to a simpler fraction are prime numbers.

An example of such fraction would be 7/10.

I swim the highest number of lengths for which this is possible.

How many lengths do I swim each week??

----- i know that prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97.. but i have problems working out word questions. please help me understand it. thank you!!