A sum of money at double the rate of simple interest doubles itself in 10 years. In how much time will that amount be triple of itself?

What will be come in place of question mark(?) in the following number series?
14, 8, 7 , 11.5, 22, ?

The smallest number that must be subtracted from 1000 to make the resulting number a perfect square is 

X and Y enter into a partnership with capital in the ratio 3: 5. After 5 months X adds 50% of his capital, while Y withdraws 60% of his capital. What is the share (in lakhs) of X in the annual profit of 6.84 lakhs?

Rahul invested equal sums of money at compound interest under two schemes A and B. Under scheme A, the interest rate was 10% per annum and under scheme B, the interest rate was 12% p.a. The compound interest after two years on the sum invested in scheme A was ₹1,050. How much is the interest earned under scheme B after two years, if the interest is compounded annually in both schemes?

A boat moves downstream at the rate of 1 km in $$7{1\over 2}$$ minutes and upstream at the rate of 5 km an hour. What is the speed of the boat in the still water?  

The HCF and LCM of two numbers are 21 and 4641 respectively. If one of the numbers lies between 200 and 300, then the two numbers are-

The value of a machine depreciates at the rate of 10% per annum. It was purchased 3 years ago. If its present value is Rs.53071.20, the purchase price of the machine was: