Question 916717
1/a+1/b=1/x
1/4+1/5=1/x
5/20+4/20=1/x
9/20=1/x
x=20/9
c) x=2.22222222 hours working together
check
x/a+x/b=1
x/4+x/5=1
2.22222222/4+2.22222222/5=1
0.55555556+0.44444444=1
1.0=1
ok
codewt

a) Alice does 1/4 in one hour
Sally does 1/5 in one hour
b) 5/20+4/20=9/20 in one hour