#### Answer

a) 0
b) 20
c) 0
d) 20

#### Work Step by Step

a) $20\times0=0$
Any number multiplied by 0 is 0.
b) $20\times1=20$
Any number times one is the number.
c) $0\times20=0$
Any number multiplied by 0 is 0.
d) $1\times20=20$
Any number times one is the number.