Centerville Bikes and Stuff $($CBS$)$ sells motorcycles and accessories. The number of helmets sold by CBS per week for the past six weeks follows.

Week $123456$

Value $171215101815$

Using the naive method $($most recent value$)$ as the forecast for the next week, compute the following measures of forecast accuracy.

$($a$)$

mean absolute error

MAE $=$

$($b$)$

mean squared error

MSE $=$

$($c$)$

mean absolute percentage error $($Round your answer to two decimal places.$)$

MAPE $=$

$\%$

$($d$)$

What is the forecast for week $7?$