Convert 3 6/2 to an improper fraction so: 12/2
convert 2 4/5 to 14/5
multiply the numerators of the improper fractions so 12*14, which is 168.
multiple the denominators so 2*5, which is 10.
the new fraction is 168/10, so you divide 168 divided by 10, which is 16.8, or 16 4/5.
X-2y=1
subtract x
-2y = 1-x
divide by -2
y = x-1
2
Your procedure is perfect, you're fine, however, bear in mind that, in a calculator when plugging in values for some functions, specially trigonometric ones, if you tell it cos(40), and the calculator is in Radian mode, it thinks you meant cosine of 40 radian units, if you give it cos(40) and it's in Degree mode, it thinks you meant 40°, and 40 radians is hugely different than 40°.
so, make sure your calculator is in Degree mode, as you'd have guessed, it isn't.
