# A Rough Guide to Everyday Mental Calculation!

 By: Murdo Macleod  We all need to check a bill from time to time. The problem is, how do you do it when you're in a rush, when you're under pressure, and when you don't have a calculator handy? Actually, it's not at all difficult if you know what to look for. Here's how to do it... Example 1 --------- Suppose you are ordering some building supplies and you want to check that the price is about right before you open your check book. You order 213 posts at \$5.85 per post. These numbers can be approximated to 200 posts at \$6 per post. Multiplying 200 and 6 is easy: the expected bill should be about \$1200. Basically you just look at the figures and round them to the nearest convenient unit. (That's why we approximated \$5.85 to \$6). Example 2 --------- Similarly for 330 pieces of turf @ 65 cents each, just find 300 × 70 = 21000 cents = \$210. --- TIP -------------------------------------------------- When multiplying numbers ending in zeros e.g. 10 x 100 = 1000: you always end up with the number of zeros added. So 1000 x 30,000 = 30,000,000 (7 zeros) --- TIP -------------------------------------------------- Example 3 --------- If there are more items in the bill you can still get a rough answer: 62 planks @ \$2.85 each, 28 joists @ \$6.99 each. Nails: \$8 Get 60 × 3 = 180 for the planks, and 30 × 7 = 210 for the joists. The nails are \$8, let's say \$10. So the rough total is: 180 + 210 + 10 = \$400 Now who needs a calculator? --------------------------------------------------------- Excerpt from 'Fun With Figures Volume 2'. Discover how to perform everyday calculations the easy way - without having to rely on your electronic friend!. ---------------------------------------------------------