How you do this resistor in circuit question?

If Resistors are in series= The equivalent is the sum.

E.g R1 and R2 in series,  R = R1 + R2.

If in Parallel, equivalent is Product/sum.

E.g If R1 and R2 in parallel,  R = (R1*R2)/(R1+R2)

1.)    60 is parallel with 40 and both are then in series with 20.

60//40   = (60*40)/(60+40) = 2400/100 = 24 

Now the 24 is in series with the 20 

R = 24 + 20 = 44 ohms.

2.)    80 is in series with 40 and both are then in parallel with 40.

Solving the series,  R = 80 + 40 =120.

Parallel:   120//40  =     (120*40)/(120+40) =  4800/160 = 30 
Equivalent Resistance = 30 ohms.

3.) 100 is in parallel with 100 and both are then in series with the parallel of 50 and 50.

The 1st parallel  =  (100*100)/(100+100) = 10000/200 = 50

The 2nd parallel  =  (50*50)/(50+50)  = 2500/100   = 25.

Solving the series  = 50 + 25 =75 ohms.

Cheers.

Iwannasay c is the most accurate