h = Harvey
j = Jane
Harvey is 3 times as old as Jane = 3 times j
The sum of their ages is 48 years.
sum = addition
The keyword is means use a equal sign so..
h + j = 48
h = 3 times j
3j + j = 48
4j = 48
4j / 4 = 48 /4
j = 12
j = 12
Now lets find for Harvey. Harvey = 3 times j
Harvey = 3j = 3 times 12 = 36
Jane = 12
Harvey = 36