If four 90 Ω resistors are connected in series across an 18 V source, the current equals ___.

By Ohm's law, V = IR.

I = V/R

Given, Four 90 ohm resisters in series. 

So, R = ( R1 + R2 + R3 + R4) = 4 x 90 = 360 ohms.

Therefore, I = 18/360 = 0.05 A [or] 50 milli amps.