let

x = the number of pages Kim read on Saturday

y = the number of pages Kim read on Sunday

Kim read 2 and 1/4 times as many pages on Saturday as on Sunday

x = (9/4)y (since 2 1/4 = (2*4 + 1)/4 = 9/4)

She read 120 pages more on Saturday

x = y + 120

by solving the system of equations

x = (9/4)y

x = y + 120

we find

x = 216 pages

y = 96 pages

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216 + 96 = 312

Kim read 312 pages altogether.