This involves creating some equations.
First, these are what the letters I used stand for:
c = children
w = women
m = men
x = total population
Then, we must make an equation for each statement.
2/5x = m (2/5 of the people are men)
3c = w (there are 3 times as many women than children)
45+c = m (there are 45 more men than children)
Now, let's start plugging in our numbers:
x = all the men, women, and children
2/5(m+w+c) = m
2/5 (45+c +3c + c) = m [Now simplify]
2/5 (45 + 5x) = m [Now Distribute]
2/5(45) + (2/5)(5x) = m
2c + 18 = m [Above we stated that there were the same amount as men as c +45]
2c + 18 = 45 +c [Now set equal to c]
2c (-c) +18 = 45 +c (-c)
c+18 (-18) = 45 (-18)
c = 27
Now that we know that there was 27 children, we can plug 27 in for c in the other equations.
MEN = 27 + 45
WOMEN = 3(27)
CHILDREN = 27
Now add those three answers to find your total!