AIEEE 2003 Physics Solved Paper

Question says, both spring balance are light that means springs are massless.The Earth pulls the block by a force Mg. The block in turn exerts a force Mg on the spring of spring balance S1 which therefore shows a reading of M‌kgf.
The spring S1 is massless. Therefore it exerts a force of Mg on the spring of spring balance S2 which shows the reading of Mkgf.