0.52 x 0.24 means "about a half times 0.24". Given the multiple choice answers, the correct answer is therefore 0.1224 (since this is the only one smaller than 0.24).
Proof:
0.50 × 0.24 = 0.12
0.01 × 0.24 = 0.0024
And therefore 0.51 × 0.24 = 0.12 + 0.0024 = 0.1224
sqrt (3/10)
sqrt(3)/sqrt(10)
no sqrt in the denominator, so multiply by sqrt(10)/sqrt(1)
sqrt(3) sqrt(10)
-------------------
sqrt(10) sqrt(10)
sqrt(30)/10
Choice D
I do A and B but that is the way I learned it. Hope this helps :)