A pair of adjacent coils have a mutual inductance of 1.5 H. If the current in one is changed from $0 \; to\; 20 A$ is $0.5 S$ , what is the change of flux with the other coil.
$(a)\; 3 wb$
$(b)\; 0.3 wb$
$(c)\; 30 wb$
$(d) 300 wb$