There are three source of resistance in parallel circuit. Two of them are rated at 20 ohms, the other at 10 ohms. What is the circuits total resistance

In paraller connection you can calculate the equivlant resistqnce from the relation:

1/R (equi) = 1/R1 + 1/R2 + 1/R3

1/R (equi) = 1/20 + 1/20 + 1/10

1/R (equi) = 0.05+0.05+0.1 = 0.2

so, R (equi) = 5 ohms