An office has 250 staff, of whom 40% work in the field. If 30 of the field staff are moved to desk work and nobody else joins or leaves, what per cent of the staff then work in the field?

Field staff = 40% of 250 = 100, and after the move 100 - 30 = 70 remain in the field. The office still has 250 staff, so the share is 70/250 = 0.28 = 28%. 12% is 30/250, the fall in the share rather than the new share itself. 52% moves the 30 the wrong way (130/250). 70% mistakes the 70 remaining field staff for a percentage - a count and a per cent are not the same thing.