The mean proportional between 25 and a number N is two times the mean proportional between 15 and 35.

Correct option is A

Formula Used:
Mean proportional between a and b = √(ab)
Solution:
$$\sqrt{25 \times N} = 2 \times \sqrt{15 \times 35} \\\sqrt{25N} = 2 \times \sqrt{525}\\\sqrt{25N} = 2 \times 5\sqrt{21}\\\sqrt{25N} = 10\sqrt{21}\\25N = 100 \times 21\\25N = 2100\\N = 84$$​​