Explanation:
a = Alice
b = Bernice
m = Mei Ling
a + b = 90.45
a + m = 61.2
a = 61.2 - m
b = 4m
so, we bring the last 2 equations into the first :
61.2 - m + 4m = 90.45
61.2 + 3m = 90.45
3m = 29.25
m = 9.75 kg
a = 61.2 - m = 61.2 - 9.75 = 51.45 kg
b = 4m = 4×9.75 = 39 kg
Alice's mass is 51.45 kg.