In a picnic there are 240 peoples,the strength of mans is 20 more then women's?

a = number of adults
c = number of children
m = number of men
w = number of women.

The number p of people attending the picnic is 240.
p = 240, and
p = m + w + c

There are 20 men more than women:
m = w + 20

Substitute m by w+20 to reduce the unknowns:
a = (w + 20) + w

There are 20 adults more than children:
c = w+20 + w - 20

c = 2w.

Thus:
m + w + c = w+20 + w + 2w = 240

Solve for w:
w+20 + w + 2w = 240
4w + 20 = 240
4 (w + 5) = 240
w + 5 = 60
w = 55

There are 55 women.

c = 2w =>
c = 110

There are 110 children.

m = w + 20 =>
m= 75

There are 75 men.