Estimating products, perimeter and ways to find area; problem solving

Corners are cut from a 40 m by 50 m rectangle to make a staircase shape. What is its perimeter in m?

Find the value of 920 − 80 : 4.

A class wants to collect 60 kg of waste paper. 5 pupils brought 6 kg each and 2 pupils 9 kg each. How many kg more are needed?

An L-shaped plot consists of parts 5 m × 4 m and 2 m × 3 m. What is its area in m²?

Find the upper bound of the product 28 × 15 (30 × 20).