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A certain sum lent out at simple interest doubles itself in 20 years. Number of years in which this sum trebles itself at the same rate of interest is

A certain sum lent out at simple interest doubles itself in 20 years. Number of years in which this sum trebles itself at the same rate of interest is
a. 30
b. 40 - ans
c. 25
d. 20
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The telephone bill of a certain establishment is party fixed and partly varies as the number of calls consumed. When in a certain month 540 calls made the bill is Rs.1800. In another month 620 calls are consumed then the bill becomes Rs.2040. In another month 500 units are consumed due to more holidays. The bill for that month would be :

 The telephone bill of a certain establishment is party fixed and partly varies as the number of calls      consumed. When in a certain month 540 calls made the bill is Rs.1800. In another month 620 calls are      consumed then the bill becomes Rs.2040. In another month 500 units are consumed due to more      holidays. The bill for that month would be : a) Rs.1560           b) Rs.1680      c) 1840            d) Rs.1950  Expl : Let the fixed amount be Rs. X and the cost of each unit be Rs. Y.     Then, 540y + x = 1800 …. And 620y + x =  2040     On subtracting (i) from (ii), we get 80y = 240 -> y = 3     Putting y = 3 in (i) we get :     540 * 3 +  x = 1800 x = (1800-1620) = 180     . :  Fixed charges = Rs.180, Charge per unit = Rs.3.     Total charges for consuming 500 units = 180 +(500*3) = Rs.1680
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