2 resistors of resistances 150 ohms and 5000 ohms respectively are in series with each other. They both are also in parallel with a 1000 ohm resistor. What is the total resistance?

Firstly, you should start by working out the total resistance for the 150 ohm and 5000 ohm resistors which are in series: -In series the total resistance is given as the sum of the resistance of each individual resistor (R_T=R₁+R₂+R₃...). -Therefore the total resistance of the 150 ohm and 5000 ohm resistors is 5150 ohms (150+5000=5150)  Imagine this total resistance of the resisitors in series (5150 ohms) being a resistor that replaces both the 150 ohm and 5000 ohm resistors. Now try working out the total resistance in parallel: -In parallel the reciprocal of the total resistance is given as the sum of the reciprocal of resistance for each individual resistor (1/R_T = 1/R₁ + 1/R₂ + 1/R₃ ...) -Therefore the reciprocal of the total resistance is 1/5150 + 1/1000 = 123/103000. So to obtain the total resistance divide 1 by this value (as it is a reciprocal) to get a resitance of 103000/123 ohms which is equivalent to 837.4 ohms (4 s.f.)