Question 590971
"A" is not the same as "a"
This is math. Be more precise.
[x/4+x/5+x/6=1
x=60/37
Together all three can do the first job in 60/37 hours. 

a does 15/37 of the first job
b does 12/37 of the first job
c does 10/37 of the first job]

second job
4.5/6=3/4
c does the job in 3/4 of the time.

3/4 of 4=3
a does the job in 3 hours
3/4 of 5=15/4=3 3/4
b does the job in 3.75 hours
y/3 +y/3.75=1
Together a and b do the second job in 1 2/3 hours