Let the number of Rs. 1's be x
Then the number of Rs. 2's, using
ONE PART = TOTAL MINUS OTHER PART,
is Rs. 50-x.
                      Value      Value
Type       Number       of         of
 of          of        EACH       ALL
coin        coins      coin      coins
-------------------------------------------
Rs. 1         x         1         1x
Rs. 2       50-x        2       2(50-x)
-------------------------------------------
TOTALS      50      -----         70

 The equation comes from the column on the right

  

1x + 2(50-x) = 70
 x + 2(50-x) = 70
  x + 100-2x = 70
    -x + 100 = 70
          -x = -30
           x = 30 = the number of Rs. 1s.

The number of Rs. 2s is 50-x or 50-30 or 20 Rs. 2's.

Checking:  30 Rs. 1's is 30 Rs. and 20 Rs. 2's is 40 Rs.
            That's 50 coins.
            And indeed 30 Rs. + 40 Rs. = 70 Rs.
Edwin