Average memory stall time per instruction = Average number of memory accesses per instruction * Average memory access time.
Assuming no memory stalls for $L_1$ hit
$L_1$ miss rate $=\frac{80}{2000 \times 1.8} = \frac{1}{45}$
$L_2$ miss rate $= \frac{40}{80} = 0.5$
Therefore, Average memory stalls time per instruction
$\qquad =1.8 \times (\frac{1}{45} \times (30 + 0.5 \times 200)$
$\qquad = 1.8 \times \left( \frac{130}{45}\right) = 5.2$ cycles