So let’s give each letter a variable

If b is 90% of c that means:
b=.9c

If a is 30% of C that means:
a=.3b

So take that second equation (a=.3b), and use the first equation to plug in for b. We get:
a=.3(.9c)
.3*.9=.27 which means:
a=.27c
.27=27%

ANSWER:
a is 27% of c

If you want to check you could give c a number.
Let’s say c=100

If b is 90% of c that means . 9(100)=b
.9(100)=90=b

To get a, multiply b by 30%
.3(90)=27=a

It checks out because 27 is 27% of 100