The Venn diagram shows the geometric regions associated with the described sets (math=maths).
KEY: SO=science only, MO=math only; SM=science & math; PASS=those that passed; PSM=those that passed both subjects; PS=passed science only; PM=passed math only; NONE=those that neither took nor passed science or math.
PSM=75
PS+PM+PSM=156 (NONE=0), PS+PM=156-75=81; PS=81-PM
PS+PSM=2(PM+PSM) (twice as many passed science as passed math), PS=2PM+PSM=2PM+75;
Substitute for PS: 81-PM=2PM+75, 3PM=6, PM=2.
So, two passed math only.