21 men can do a piece of work in 35 days by working 9 hrs a day. But 15 women can do it in 27 days by working 7 hrs a day. Then the ratio of the work done by 1 man and 1 woman is:

3 : 7

Total Work = No. of person × days

Therefore,

Total work ⇒      21M × 35 x 9  = 15W × 27 x 7

                ⇒            M : W       =  \(\frac{15\; \times\; 27\; \times\; 7}{21\; \times\; 35\; \times\; 9}\)

⇒   M : W = 3 : 7