d is certainly a lot easier than b. Start by understanding that 12 = 6 + 6 which you should be able to handle in your head.
6 * 25 + 6*25
150 + 150
300
This is a prized skill. You should practice it all the time. It leads to estimation which is something you should try to do in any math class you take.
B
How about 10 twenty-sixes + 1 twenty-six. Let's see how that works out.
10* 26 + 26
260 + 26
286 You should check to see if that works. Notice that the 2 and the 6 add to 8 which is what they should do if you are multiplying by 11. The end numbers add to the middle one for 3 digits. It get's a little messy when the end two number add to something greater than 10, but that's another story.
