The student scores:

• Hindi: $120\times 0.40=48120\; \backslash times\; 0.40\; =\; 48$120×0.40=48
• English: $140\times 0.55=77140\; \backslash times\; 0.55\; =\; 77$140×0.55=77
• Mathematics: $100\times 0.45=45100\; \backslash times\; 0.45\; =\; 45$100×0.45=45

Total marks obtained: $48+77+45=17048\; +\; 77\; +\; 45\; =\; 170$48+77+45=170.

To achieve 60% in total $540540$540 marks, he needs $324324$324 marks. Thus, in Science:

$x+170=324\Rightarrow x=154.x\; +\; 170\; =\; 324\; \backslash Rightarrow\; x\; =\; 154.$x+170=324⇒x=154.

He should score 154 marks in Science.