If Dev can make 40 chairs in 20 days, working 10 hours per day then in how many days can he make 10 chairs working 8 hours per day?

$6\frac{1}{4}$

Formula to be used here,

⇒ \(\frac{D_1\;×\;H_1}{D_2\;×\;H_2}\) = \(\frac{W_1}{W_2}\)

where, D = No. of Days, H = No. of Hours, W = Work

⇒ Putting values in the formula,

⇒ \(\frac{20\;×\;10}{D_2\;×\;8}\) = \(\frac{40}{10}\)

⇒ \( { D}_{2 } \) = \(\frac{25}{4}\) = \( { 6}_{ 4}^{ 1} \) days,