here's another answer. These story problems are all basically the same: translate words to symbols and solve the equation(s).
m = 4s
m = 5d
now, we need to figure out how many years from now the son will be 3 times his present age:
3s - s = 2s = m/2
so, adding that many years to mother and daughter, we have
m + m/2 = 2(d+m/2)+3
so, we now have two equations
m = 5d
3/2 m = 2d + m + 3
3/2 (5d) = 2d + 5d + 3
15/2 d = 2d + 5d + 3
d/2 = 3
d = 6
in 15 years we will have
s = 22.5
d = 21
m = 45 = 2*21 + 3
Now try some of your other problems, and show us where you get stuck, if you do.
present: let .......
mother ---20 x
son ------ 5x
What is 3 times son's age ---> 15x
When is son 3 times as old as he is now?
in 15x - 5x years or in 10x years
at that time
son ----- 15x
mother --- 20x + 10x = 30x
daughter -- 4x + 10x = 14x
mother's age = 2 times daughter's age plus 3
30x = 2(14x) + 3
2x = 3
x = 1.5
mother -- 30
son ----- 7.5
daughter -- 6
3 times son's present age is 22.5, this will happen in 15 years
at that time ...
mother is 45
son is 15
daughter is 21
is 45 = 2(21) + 3 ??? , YES!!
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