(1-(1/5+1/7.5)×2)÷(1/7.5)
= (1-(3/15+2/15)×2)÷(2/15)
=(1-1/3×2)÷2/15
=(1-2/3)÷2/15
=1/3÷2/15
=1/3×15/2
=5/2