If R1 and R2 in series give a total resistance of 12,ohms,calculate R1 such that their combined resistance in parallel is a quarter of R1

Answer 1

1/R = 1/R1 + 1/R2
1/(R1/4) = 1/R1 + 1/(12-R1)

R1 = 9
R2 = 3

check:

1/(9/4) = 1/9 + 1/3
4/9 = 1/9 + 3/9
yes!