a skinny girl devours food that is equivalent to 600 kcal, as she is anorexic she repents and wants to lose twice as many calories by lifting 20 kg weights to a height of 1.9 m. If she measures 1.7m ... she calculates the number of times she must lift the weights to lose the same amount of energy that she acquired with food and the time she must be exercising if it takes 2 seconds for repetition.

Explanation:

Work done in lifting the weight once = mgh

= 20 x 9.8 x (1.9+1.7)

= 705.6 J  

= 705.6 / 4.2 calorie

= 168 cals

Total energy to be spent = 600 x 10³ cals

No of times weight is required to be lifted

= 600 x 10³ / 168

= 3.57  x 10³ times

Total time to be taken = 2 x 3.57 x 10³

= 7.14 x 10³ s

=119 minutes .

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.