In an examination, 80% passed in English, 70% in Mathematics, and 10% failed in both. If 144 students passed in both, total students: MCQ with Answer and Explanation

Explanation:
0.8x = 64 → 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.

Explanation:
Milk=40 L, water=20 L. Withdraw x L mixture: milk removed = (2/3)x, water removed = (1/3)x. After replacement, milk = 40 - 2x/3, water = 20 - x/3 + x = 20 + 2x/3. Ratio 1:2 → (40 - 2x/3)/(20 + 2x/3) = 1/2 → 2(40 - 2x/3) = 20 + 2x/3 → 80 - 4x/3 = 20 + 2x/3 → 60 = 2x → x = 30.