Maths questions. Radians/indices/volumes?

Question

I've been working on my homework but I have no one to check it. Could someone please just confirm if my answers are correct and if not, advise me of the correct THEORY as I really want to learn. Thanks!

1. Simplify in index form
a) a^4 x a^5 / a^2
b) (p^2 x q^3 x r^4 (p^2)^3) / pqr
c) Q^3 x Q^-4 x Q^-5

My answers:
a) a^7
b) p^7q^2r^3
c) Q^-6

2. Evaluate:
a) 6^2 x 5^2
b) 121^-1/2
c) 0.5^3 + 0.25^3 x 0.75^3

My answers:
a) 900
b) -11
c) 0.059

3. A beach balls diameter is 1.27 x 10^4 km and a tennis balls diameter is 6.79 x 10^3 km
By calculating their volumes find how many times bigger one planet is to another.

My answer:
I got the beach ball is six times bigger

THANK YOU SO MUCH x

Answer 1

1a) a^4 *(a^5/a^2)
=a^4*(a^{5-2})
=a^{4+3}
=a^7

b) correct
c) correct

2a) 6^2 *5^2
=36*25
=900

b) 121^-1/2
= 1/ (root 121)
= 1/11

c) 0.131591797

Now i'm too tired to anyore do maths!...

Answer 2

2b is 1/11
2c is .5625

Vb = 4/3 * pi * ((1.27/2)*10^4)^3
Vt = 4/3 * pi * ((6.79/2) *10^3)^3
Vb/Vt = 2.56047875 x10^12/3.913085488 X10^10
Vb/Vt = 6.543376