4 workers working 6 hours per day can finish painting a wall in 21 days. If 7 workers work for 4 hours per day, in how many days will they be able to finish the same work?

The relation between

Number of workers(N) and work is directly proportional,

Number of working hours(T) and work is directly proportional.

Number of days(D) and work is directly proportional.

If 7 workers work for 4 hours per day,  let x be the number of days to finish the same work.

$$N_{1} \times T_{1} \times D_{1}$$ = $$N_{2} \times T_{2} \times D_{2}$$

$$4 \times 6 \times 2$$ = $$7 \times 4 \times x$$

x = 18

So , the answer would be Option c) 18 days.