1+-1/15*-1/20 = 3/7-6/7*1/3=

1+-1/15*-1/20 = 1.003333333 3/7-6/7*1/3= 0.1428571429

1 + (- 1/15) x 1/20
= 1 - 1/300
= 300/300 - 1/300
= 299/300

3/7 - 6/7 x 1/3
= 3/7 - 6/21
= 9/21 - 6/21
= 3/21
= 1/7