Solve x? - 8x + 15 <0

The capulet and montague families love writing. last year, each capulet wrote 444 essays, each montague wrote 666 essays, and both families wrote 100100100 essays in total. this year, each capulet wrote 888 essays, each montague wrote 121212 essays, and both families wrote 200200200 essays in total. how many capulets and montagues are there? choose 1

Can someone explain to me how to do

1) prove that 731^3−631^3 is divisible by 100 2) prove that 99^3−74^3 is divisible by 25