We need to solve the problem by algebra which is taught in most schools.
In algebra, most of the time, we call the number we're looking for as "x", which we don't know yet. Using "x", we formulate an equation according to what is given, which is O has 32 stamps, and S has 154.
When each of them bought x stamps, O has 32+x, and S has 154+x.
We're also told that S now has 3 times as many as O, so the equation goes
154+x = 3(32+x) .......(1) from which x can be solved.
We first expand both sides.
154+x = 96+3x (called the distribution property)
Now subtract x from each side,
154+x-x=96+3x-x => 154 = 96+2x
Now subtract 96 from each side, and simplify as above
154-96 = 2x
simplify again
58=2x
divide by two on each side,
x=29.
Let's check: 3*(32+29)=3*(61)=183 = 154+29, so the answer is correct.
Since they bought EACH 29 stamps, the bought a total of 2*29=58 stamps (which happens to be the difference of number of stamps at the beginning).
