Step 1: Selecting R

Since the above problem requirement suits for monostable multivibrator. To obtain 20 seconds delay to keep LED on we need to adjust the values of RA and C which is obtained from following equation, $$T_b = 1.1R_AC$$ Given data :

$T_b$ = 20 seconds

Assume C=1µf $$20 = 1.1 R_A × 1µf$$ $$R_A = \frac{20}{1.1× 1µf}$$ $$\boxed{R_A = 18.18MΩ}$$

Step 2 : Designed Circuit Diagram