Question 1183664
 Bill is 5 times older than Keira,
b = 5k
 Bill is 5 times older than Phil,
b = 5p
 Oscar is twice as old as Bill, 
o = 2b
 and the total age of all of them at 81.
b + k + p + o = 81

Something wrong here, as written, this will not produce an integer solution which is generally required in problems like this.
Change a couple words in this problem, it works then
"
Bill is "3 years" older than Keira,
b = k + 3
k = b - 3
 Bill is 5 times older than Phil,
b = 5p
divide both sides by 5
p = .2b
 Oscar is twice as old as Bill, 
o = 2b
 and the total age of all of them at 81.
b + k + p + o = 81
 replace: k with (b-3), replace p with .2b and o with 2b
b + (b-3) + .2b + 2b = 81
4.2b = 81 + 3
b = 84
b = 84/4.2
b = 20 yrs old is bill
then
k = 20-3 = 17 yrs is Keira
and
p = .2*20 = 4 yrs is Phil
and
o = 2 * 20 = 40 yrs is Oscar
:
Check: 20 + 17 + 4 + 40 = 81
:
Anyway this is the method.