algebra 1
posted by Robbie on .
plz explain how to do this in detail:
using only 39 cent and 24 cent stamps, charlie put $7.14 postage on a package he mailed to his sister. if he used twice as many 39 cent stamps as 42 cent stampsp, how many 24 cent stamps did he use??

first represent the unknowns using variables:
let x = number of 24cent stamps used
let 2x = number of 39cent stamps used (according to the second statement)
since the total is 7.14 dollars,
0.24x + 0.39(2x) = 7.14
now we solve for x:
0.24x + 0.78x = 7.14
1.02x = 7.14
x = 7 (number of 24cent stamps used)
hope this helps~ :) 
Triple a number plus 7 results in 16. What is the number?

let x = number
according to the first statement, we multiply the number by three, then we add 7 to get 16,, writing in equation:
3x + 7 = 16
now solve for x,, first we transpose the 7 on the other side of equation,, to do this we change its sign from positive to negative:
3x = 16  7
3x = 9
to get x alone, we divide both side of equation by 3:
(3x)/3 = 9/3
x = 3
hope this helps~ :)