substraction field type time

I'm a beginner php-mysql programmer,, and i have some problem,, 1 of them is ....

i have a table "gangguan83" with field

urut_gangguan int(15) -> primary key.
lv int(10),
trip datetime,
masuk datetime,
current int(10),
ens int(10),
....

how can i query if m开发者_Go百科y statement like this :

ens = (lv * 1,73 * 0,9 * current * (masuk-trip))

please help me. Thanks very much before.

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SELECT lv * 1.73 * 0.9 * current * (masuk - trip) AS ens
...