You mistakenly added what should have subtracted 0.42 from a number, and ended up with 0.9. What is 0.5 added to the correctly calculated value?
0.9 0.42 [OP_SUB] 0.42 [OP_SUB] 0.5 [OP_ADD]
var_a = 0.9
var_b = 0.42
var_c = var_a - var_b
var_d = 0.42
var_e = var_c - var_d
var_f = 0.5
var_g = var_e + var_f
print('{:.2f}'.format(round(var_g+1e-10,2)))