Statistics Question, please explain steps

Question

Please explain all steps, thank you. Which formula is used?

Why is it not 0.82(0.7) + x(0.9) / 0.7 + 0.9?

Answer 1

82 is worth 70%

If she got 100 on the remaining 30% her final grade would be .7(82 +.3(100+ 57.4 + 30 = 87.4 < 90

.7(82) + .3x = 90

.3x = 90-57.4 = 32.6

x = 32.6/.3 = 108 2/3

she needs a grade of 108 2/3 to get a 90

if they round off up for .5 or more

then

x= (89.5 -57.4)/3 = 32.1/3 = 107

she needs a 100 plus 7 points extra credit to get an 89.5 which usually rounds up to 90