2/15÷（19/21+3/7) /(1/2)
= 2/15 ×(1/2) /（19/21+3/7)
= 2/15 ×(1/2) /{（19+9)/21}
= 2/15 ×(1/2) /{28/21}
= 2/15 ×(1/2) /(4/3)
= 2/15 ×1/2 ×3/4
=1/20