dr. potter provides 54 vaccinations TOTAL (including both measles and polio).
p+m=54
since there are 4 doses of polio and of measles, you have to use a variable to distinguish how many doses were given for measles and polio out of 142 given doses.
4p+2m=142
p+m=54
4p+2m=142
^^system of equation
p+m=54
4p+2m=142
p=54-m
4p+2m=142
(substitute 54-m to the equation)
4(54-m)+2m=142
m=37
(collect like terms, then substitute the value of m into the first equation)
