20-day income: x*( 1.0-0.9)*20=2x.

/kloc-income within 0/0 days: [250 * 0.1-(x-250) * (0.9-0.1)] * 10 = 2250-8x.

Total income: 2250-6x

So x takes the minimum value of 250, and the total income is 750 yuan.