Answer:
N + B + D = 90 sum of ages is 90 (I)
B = 2 D B is twice as old as D
N + 3 D = 90 using above equations
(N + 10) + 3 D = 100 add 10 to both sides
B + 3 D = 100 Brian is 10 yrs older than Nigel
2 D + 3 D = 100 Brian is twice the age of Dean
5 D = 100
D = 20 Dean's age
Brian = 2 D = 40 Brian's age
90 - 60 = 30 = Nigel's age
Check:
N + 10 = B = 30 + 10 = 40 = Brian's age