Not exactly
2/10 = .2
1/4 = .25
1/4 is 0.05 bigger than 2/10
Try creating a proportion, take 12/21=x/15
you should have gotten x=8.571
P.S.: hope I helped
Refer to the figure below.
Assume that there are n correct measurements

Because each measurement has an error of 0.2 in, the actual measurements are

From the figure, the analysis indicates that the correct mean is
μ = 3 in = 3*2.54 cm = 7.62 cm
The correct standard deviation is
2.934 in = 2.934*2.54 cm = 7.45 cm
Answer:
μ = 7.62 cm
σ = 7.45 cm
2.8y+6+0.2y=5y-14
Combine like terms
3y+6=5y-14
Subtract 6 from both sides
3y=5y-20
Subtract 5y from both sides
-2y=-20
Divide both sides by -2
Y=10