Bonjour,
a) 1/(1/15) = 1 * (15/1)
= 15
b) 1/(1/1.35) = 1 * (1.35/1)
= 1.35
c) 1/(1/19) = 1 * (19/1)
= 19
d) 1/(1/(-8)) = 1 * (-8/1)
= -8
e) 1/(7/4) = 1 * (4/7)
= 4/7
f) 1/(-19/20) = 1 * (20/(-19))
= 20/(-19)
g) 1/(6.2/3.4) = 1 * (3.4/6.2)
= 3.4/6.2
h) 1/(-7/12) = 1 * (-12/7)
= -(12/7)
Voilà.