0 because nobody like history
jk
assuming that she spends all of her time on those 3 subjects
all-math-science=history
all=1
1-1/3-1/4=history
make bottom number same
34=12, so bottom number has to be 12
1/34/4=4/12
1/4*3/3=3/12
1=12/12
12/12-4/12-3/12=history
(12-4-3)/12=history
(12-7)/12=history
5/12=history
spends 5/12 of her timem on history
