In an athletics team, tall members are three times more likely to win a medal than short members. this year the team has won 60 medals so far. how many of these have been won by short athletes?

Well lets say y = the amount of medals won. lets say x is the amount won by short people and z is the amount won by tall. we know that the total medals is the total medals won by short and tall so:
y = x + z

it says that tall people are 3.times more likely. this would mean they won three times more than short people. so
z = 3x.

so if we know what z equals, we can replace z with what it is equal to. so:
y = x + z
y = x + 3x
y = 4x

we know that 60 medals were won so
y = 4x
60 = 4x
x = 15

that is the amount won by short people