Let's say that the money that each person has is equal to a letter:
A=Debbie, B=John, C=David.
Now we can write some equations.
"John has four times as much money as his sister Debbie who has half as much as their friend David."
B = 4A
A = 0.5C
"Together the three children have $105"
A+B+C=105
This is now a system of equations problem. Substitute to solve.
A + (4A) + C = 105
(0.5C) + 4(0.5C) + C = 105
3.5C = 105, so C = 30.
Plug this into the previous equations to get A=15 and B=60
So Debbie has $15, John has $60, and David has $30
