Question 842595
formulate an equation out of the description. 
What do we know:
1) there are 40 biscuits 
2) there are 3 children
3) B has four more than A which said another way whatever A has, B has 4 + A's 
4) C has twice as many as B's which is the same as saying 2 * (4 + A's)

when we put all this together we have a simple equation with only one unknown. lets call A's number of biscuits x to be clearer so that

x + (4+x) + 2(4+x) = 40; this is saying A's biscuits + B's biscuits + C's biscuits = 40

solve for x by simplifying first

2x + 4 + 8 + 2x = 40
4x + 12 = 40
4x = 40 - 12 = 28
x = 7 = A's biscuits

go back to equation 3 so that B's biscuits = 7 + 4 = 11
go back to equation 4 so that C's biscuits = 11 * 2 = 22

check your answer is 11+22+7 = 40?

Q.E.D