The contribution of the students amounted to $5 x 20 = $<<5*20=100>>100.
So Miss Grayson's class had $100 + $50 = $<<100+50=150>>150 in all.
The cost of the field trip amounted to $7 x 20 = $<<7*20=140>>140.
Therefore, the class of Miss Grayson is left with $150 - $140 = $<<150-140=10>>10.
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