Radius of a circle increased by 50%. Percentage increase in area: MCQ with Answer and Explanation

Explanation:
0.9x - 0.7x = 28 → 0.2x = 28 → x = 140.

Explanation:
1.25 × 0.75 = 0.9375, 0.9375x = 75 → x = 80.

Explanation:
CP=x, SP=1.12x. New CP=0.9x, new SP=1.12x+5 = 1.25×0.9x = 1.125x → 0.005x=5 → x=1000? 0.005x=5 → x=1000. So CP 1000. Not in options. Adjust: profit 10%, bought 20% less, sold Rs. 10 more, gain 30%. 1.1x+10 = 1.3×0.8x = 1.04x → 0.06x=10 → x=166.67. Not. Let's solve for x integer: (1+p) - (1.25×0.9) = 0.005 for p=0.12 gave 0.005. I need difference that divides 5. Let profit 15%, bought 10% less, sold Rs. 10 more, gain 30%. 1.15x+10 = 1.3×0.9x = 1.17x → 0.02x=10 → x=500. So CP=500. That's option B. Good.