The efficiency of the block and tackle system can be calculated using the following steps:

1. Mechanical Advantage (MA):

   The mechanical advantage is the ratio of load (W) to effort (E):

   $$MA = \frac{W}{E}$$

2. Efficiency (η):

   Efficiency is the ratio of mechanical advantage to the number of pulleys (n), multiplied by 100:

   $$\eta = \frac{MA}{n} \times 100\%$$

Given that the load (W) is 80 kgf and the effort (E) is 18 kgf, we can calculate the mechanical advantage:

$$MA = \frac{80}{18} = 4.44$$

Since there are 5 pulleys in the system, the efficiency is:

$$\eta = \frac{4.44}{5} \times 100\% = 88.89\%$$

Therefore, the efficiency of the block and tackle system is 88.89%