You can either do;
1. 90(2) = 100(2)
180 = 200
In 3 minutes, they can write 200 words. So now we can divide by 3.
3/3 = 1 minute
200/3 ≈ 66.666
or
2. 90/3 = 30 secs
100/3 ≈ 33.333
30(2) = 1 minute
33.333(2) ≈ 66.666
Either way, the admin assistant can write 67 words per minute. (Rounded to the nearest whole)
