Problems Type 1:
A can finish work in X days.
B can finish work in Y days.
Both can finish in Z days = (X*Y) / (X+Y).
Problems Type 2:
Both A and B together can do work in T days.
A can do this work in X days.
then, B can do it in Y days = (X*T) / (X-T)
Problems Type 3:
A can finish work in X days.
B can finish work in Y days.
C can finish work in Z days.
Together they can do work in T days = (X*Y*Z)/ [(X*Y)+(Y*Z)+(X*Z)]
Problems Type 4:
A can finish work in X days.
B can finish work in Y days.
A*X = B*Y
Y = (A*X) / B
Problems Type 5:
A can finish work in X days.
B can finish work in Y days.
A left work after P days then B need to work Q days
Q = [(X - P) / X ] * Y
Problems Type 6:
M1 person can do W1 work in D1 days and H1 hours
M2 person can do W2 work in D2 days and H2 hours
M1D1H1 = M2D2H2