4. A mixture of 20 kg. of spirit and water contains 10% water. How much water must be added to this mixture to raise the percentage of water to 25% ?

5. A grocer buys two kind of rice at Rs. 1.80 and Rs. 1.20 per kg, respectively. In what proportion should these be mixed, so that by selling the mixture at Rs. 1.75 per kg., 25% may be gained ?

6. Some amount out of Rs. 7000 was lent at 6 % p.a. and the remaining at 4% p.a. If the total simple interest from both the fractions in 5 years was Rs. 1600, the sum lent at 6% p.a. was :