This can be answered using multiplication.
90 X 6 = 540.
A strategy to solving this problem is to ignore the "zero" on the 90 and just multiply 9 X 6 = 54. Then add the zero back on for an answer of 540 calories.
This can be used with any number ending in zero/zeros.
A candy bar has 50 calories. How many calories in 5 candy bars?
5 X 5 = 25. Then add the zero for an answer of 250 calories.
A candy bar has 500 calories. How many in 5 candy bars?
5 X 5 = 25. Then add the two zeros and the answer is 2,500 calories.