You can use the 2-point form of the equation for a line to work this out. If you have a number between 24 and 75 and you want to find the corresponding percentage, your two points are
(24, 0), (75, 1)
and the equation for the line is
y = (x-24)*(1-0)/(75-24) + 0
y = (x-24)/51
If you want to convert to a percentage, this becomes
y = (x-24)/51*100%
If you have a percentage between 0 and 100% and you want to find the corresponding number, solve the above for x.
Y*51/100% = x-24 (divide by the coefficient of x)
x = 24 + 51y/100% (add 24)
(24, 0), (75, 1)
and the equation for the line is
y = (x-24)*(1-0)/(75-24) + 0
y = (x-24)/51
If you want to convert to a percentage, this becomes
y = (x-24)/51*100%
If you have a percentage between 0 and 100% and you want to find the corresponding number, solve the above for x.
Y*51/100% = x-24 (divide by the coefficient of x)
x = 24 + 51y/100% (add 24)